To pick a power supply, add all the positive current requirements for the modules you want together. For example, using the table below, if you have a DSC2000, a KW2010 and a FD2020, the total is 87 mA. .

The PS500 will supply 500 mA (+15V @ 500 mA, -15V @ 500 mA).

The average module current draw is around 50 mA. As a quick rule of thumb, figure 10 modules per PS500. (If you have say, two TM2050s, however, you are not typically going to be able to run 8 additional modules.)

A more complete list of power requirements for each module is in the right hand column of the credit card order page:
 
DSC2000
12 mA
KW2010
60
FD2020
15
FF2030
57
MX2040
35
TM2050
95
EG2070
41
MW2090
65
VCO2100 55
VCA2200 47
Multi2210 passive
IO2225 35