How many thalamocortical relay cells synapse onto each spiny stellate cell in neocortex? I am curious about how many different thalamocortical relay cells synapse onto each layer IV spiny stellate cell, on average? The answer is likely to be different per region and species, of course. I understand the topographic mapping that occurs, but how narrow or wide is the final projection?
From Shepherd's Synaptic Organization of the Brain (5th ed, pg 513):


  In the cat visual cortex, the terminal arbors of each individual
  thalamic afferent may extend over 1-5 mm of the cortical surface (Fig. 12.11) 
  so that each point in layer 4 is covered by the arbors of at least 1000 
  separate thalamic relay cells. Thus, the dendritic tree
  of an average layer 4 neuron, which extends for 200-300 micron, could
  receive input from many more thalamic afferents. However, the
  connections are not made randomly between the geniculate afferents
  and the cortical neurons. Selectivity is expressed in several ways.
  For example, there is a high degree of precision in the visuotopic map
  recorded in the first-order cortical neurons in the input layer, i.e.,
  those receiving mono-synaptic activation by the thalamic afferents.
  This clustering is made according to the eye preference of the arbors.
  The afferents of those thalamic relay neurons that are driven by the
  right eye cluster together in regions about 0.5 mm in diameter and are
  partially segregated from the afferents that are driven by the left
  eye. This segregation forms the basis of ocular dominance columns.  


There are, of course, many other factors in the specificity of thalamocortical projections.
Point here being that thalamocortical projections are not necessarily limited to cortical neurons with a shared receptive field in visual space, but may be limited in other ways.  

Interestingly, the corticothalamic feedback projection is also not limited to shared spatial receptive fields (Shepherd, p. 321ff.):


  Thus, for the lateral geniculate nucleus, this cortical pathway comes
  from visual cortex (mostly areas 17, 18, and 19), and likewise,
  somatosensory and auditory cortex project back, re- spectively, to the
  ventral posterior lateral and medial geniculate nuclei. One
  implication of this reciprocity is that the corticothalamic pathway
  faithfully ad- heres to the map established in the thalamic nucleus.
  For instance, the corticogenicu- late pathway conforms to the
  retinotopic map in the lateral geniculate nucleus. However, there is
  some question as to the extent to which the maps match at the cellular
  level. This is based on evidence that, in the cat (Murphy and Sillito,
  1996), the spread of an individual corticogeniculate axon arbor can be
  quite extensive, reaching well beyond the region within which
  receptive fields that match those of the cortical axon can be
  recorded. The corticogeniculate terminals have a maximal extent of 1.5
  mm compared with the spread of a typical retinogeniculate arbor of
  only about 0.2-0.4 mm (Bowling and Michael, 1984; Sur et al., 1987).
  The retinogeniculate arbor's expanse roughly corresponds to the size
  of a geniculate receptive field, implying that the corticogeniculate
  axonal arbor can contribute to subtle effects on relay responses
  beyond the "clas- sic" receptive field. However, the majority of the
  corticothalamic terminals lie in a central core that roughly
  corresponds to the classical receptive field.