hi Sadie,
ref your initial question there are a number of things to factor.At the start of the network rollout a certain number of sites and hence TRUs (radios) will have been budgeted for. However this is theoretical and (99% of the time) inadequate.
Also as stated by Sheetanshu a number of BSCs will have been calculated and will limit the amount of TRUs that can be supported. This usually affects things later on depending on traffic take up.
On a site basis then there are limits from the vendors on how many TRUs can be configured. high capacity (traffic) sites usually configured as 12/12/12 for example which is a total of 36 TRUs. Low traffic sites (rural) may only have one TRU (1/0/0). This assumes a standard 3 sectored site and no co located BTS.
Bottom line is that as the network grows and traffic increases/decreases then individual sites SHOULD be optimised to have the required number of TRUs for the amount of traffic it is taking. This can be done from network stats (usually BSC/BTS monitoring). These can be applied to the erlang table for the required GOS and the number of TRUs can be worked out.
Calculation of number of TRUs per BSC should be done very carefully as the traffic for a BSC does not relate geographically to where that traffic is.
Also as each TRU has a frequency then obviously the less used the better from an interference point of view. Also financially the physical TRU costs money AND so does the features (ie hopping implementstion etc).
Hope this helps a little