Using Fuses to Connect Batteries in Series
Fuse ratings in a design should be set so they interrupt the current for an over-current condition. You determine how much current your design needs. If your load cannot tolerate large currents or there are extreme safety requirements, you may want many fuses. They should be rated so they won't trip during normal use, but trip if something shorts out. This really really depends on the load so follow these steps:

1) Find your loading current (If its an active load like a motor then experimentally find the nominal current with a power supply) 

2) Give yourself some margin, if your load is drawing 2A normally, then multiply that by a saftey-factor so that if there is a voltage spike it won't blow the fuse out. A 50% saftey-factor would give you 3A (of not tripping). If the load shorts (lets say 1Ohm worst case) Then for a 50V pack you would get 50V/1Ohm = 50Amps).  So you would need a fuse that blows with less than 50Amps of current, but give yourself some margin in that direction too, and maybe get a fuse that trips at 10Amps.  

3) Find a fuse with the rated voltage and current, keep in mind every fuse takes time to blow or trip, will the load or source have a problem with that? If the fuse takes 1 second to blow, its probably not going to protect the batteries, if it takes 1us it probably will because not much heat will be dissipated anywhere. 

Edit:
4) If you have a captivate load, you will want to watch the inrush current. (Li-pos are "kind of" capacative in the fact that they are a very low ohmic source and their chemsitry can deliver lots of current fast.) There are slow blow fuses that are more suited to this sort of thing. Double check the inrush rating and design for it.