Hmmmm?
I don't suppose there are any theorhetical limits to the number of users you can support, or at least not any that you can reach with the hardware you are likely to employ.
More likely, the question should be how many emails can my system send in a day... then you can calculate how large the list is and how many times you expect folks to use the list.
What are you doing that needs a big list?
Have you tried asking the EZMLM people? ezmlm.org