How many customers can be connected to a Single Wire Earth Return node/pole in a power grid
SWER power and customer capacity depends on a number of factors.    

What country is this in? - important due to regulatory aspects.
Are the 200-300 customers served by a single SWER line with a single feed point or are there multiple lines and feeds. If there are multiple lines, how many customers maximum are liable to be connected to a single line. 

Capacity per 19 kV SWER line: 

In some administrations maximum current is limited by regulatory restrictions amed at limiting interference with telecommunications systems (due to the purposeful 'injection' of all line current into the ground causing vastly more impact on telecom systems than do balanced multiwire power circuits.) In New Zealand maximum current was at one stage limited to 8A (may be different now).
An 8A,19 kV system can nominally accept 150 kVA - with output being constrained by losses - which vary with line length and soil resistivity, and system power factor.  

SWER line customer capacity 

Tony suggested a 25 residence capacity per distribution transformer in typical residential use. In this application if the power input was 150 kVA  then assuming say 125 kVa delivered power (probably high), that would allow 5 kVa simultaneous peak load per user. While that level of usage is conceivable for users who understood the nature of the system and the need to minimise load, especially during peak periods, it is well below what might be drawn in some domestic situations.

In SWER systems per residence demand is usually restricted to far lower levels than in urban multi phase systems. A mean diversified load of 3 kVa / dwelling is typical in western use and in developing country applications with new electrification mean maximum loads of 1 kVa may be reasonable.

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An extremely good site for SWER related information is Rural Power  who are highly SWER focused. 
The site is fronted by a NZ based engineer, Conrad Holland who notes - " ... qualified in power engineering and has been active in rural electrification in Africa, Asia, Australia and New Zealand for 25 years."

From their SWER FAQs 3


  To calculate the single phase transformer size you must multiply the amount of customers within about a 600 to 1000 metre radius of the transformer by 600 to 1000 VA for newly electrified customers in rural areas e.g. if there are 60 households in a small village that has never had electricity before, then you would multiply 1000 VA x 60 houses = 60,000 VA or 60 kVA. The standard transformer sizes are approximately 50 kVA, 63 kVA and 100 kVA therefore you would probably install a 63 kVA transformer. In New Zealand the average household has an ADMD (after diversity maximum demand) of about 3000 VA, however a newly electrified village would have a lower demand as each household would probably only have lighting, TV etc for the first 5-10 years after this and as the economy improves they would buy freezers, air conditioners, washing machines etc and the load would increase. When the load increased you would install either a larger transformer or another transformer.


From their site - excellent SWER technical comments


  What is SWER?
  SWER, How does it work?
  Low Voltage Service Drop
  Low Voltage Pole Top Configuration for Rural Electrification
  Distribution Transformer for Rural Electrification using SWER
  SWER Inline Strain Pole for Rural Electrification
  SWER Large Angle Pole for Rural Electrification
  Small Angle Pole for Rural Electrification Using SWER
  SWER Isolating Transformer
  SWER Inline Pole  
  
  


Here is an extremely good SWER overview - when one wire is enough. This contains substantial technical information re typical line lengths, customer density, allowed loads, distribution and feed transformer sizing, earth impedances and line loses and more. 
They note:


Studies allowed 2% for low-voltage regulation and assumed the use of 16- and 25-kVA, 19,000-kV/500-250-V transformers with an approximate 4% impedance and taps of ± -5%, 2.5% and 0%.)
Load densities. Load densities for a SWER distribution system typically are less than 0.5 kVA per kilometer (0.31 kVA per mile) of line with a maximum demand per customer of 3.5 kVA. A large system may supply up to 80 distribution transformers with unit ratings of 5 kVA, 10 kVA and 25 kVA. The load patterns and demands vary greatly from customer to customer and from one season to another; thus, as load growth continues, SWER systems are reaching their technical capability. And with customers keenly aware of supply quality, customer complaints are increasing.


Slideshow - 16 slides Practical SWER design  - excellent (pdf)

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Superb 427 page World Bank pdf - 2006:
Sub-Saharan Africa - Introducing Low-cost Methods in Electricity Distribution Networks - ESMAP Technical Paper 104~06 October 2006

Sounds relevant :-)


The level of electrification in sub-Saharan Africa is low, with less than 10 percent of the rural households
having access to electricity. One of the key barriers to accelerating access is the high cost of connections,
arising, inter alia, from the use of outdated, unsuitable, high-cost methods in electricity networks. A second
key barrier is the small and dispersed nature of electricity demand, arising from low density of population
and low income levels, which lead to high average costs of providing electricity service. The objective of this
report is to help in reducing the high costs of electrification by documenting proven, low-cost methods and
techniques in electricity networks that have not yet become widely used in sub-Saharan Africa.


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"Alternatives" - Papers on a wide range of non national-grid energy sources and applications in developing countries - Biomass, solar, charcoal, micro-hydro, biogas, sisal residues, Tanzania - solar home systems,  wind, hydrothermal, ... PLUS regulatory, modelling, finance, access, sustainability, ... . 

MICRO PERSPECTIVES FOR DECENTRALIZED ENERGY SUPPLY   - Proceedings of the International Conference Technische Universität Berlin, 7th-8th of April 2011. 309 page pdf. 

For interest only - URL of the above is: http://d-nb.info/1010874136/34

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Related:

Abstract - Extending SWER line capacity

Abstract Capacity Enhancement for Aging Distribution Systems using Single Wire Earth Return

Ad but ... S&amp;C’s Energy Storage Solution Corrects Line
Resistance in Australia’s Outback

MSc thesis 1972 - looks good AN ANALYSIS OF SINGLE WIRE EARTH RETURN(SWER) SYSTEM
FOR RURAL ELECTRIFICATION