Pete,
I have both the 24 & 12 Terminal Blocks and Barry is correct, however my 24 post Terminal Block had 20 sets of wires running to it, when I thought I would have had much less. Remember this all depends on how you are planning to wire your layout from the outset. However if you are going to star wire a medium sized layout, the 12 terminal Block is more than adequate. Myself I always over power and over Terminal Block my layouts.
PCRR/Dave