[Zope] large number of users in standard acl_users ?

Hi, is it possible to store a large number of users in a standard acl_users folder, provided that very few are connected at the same time ? I'd want to store around 20000 user ids there, all six characters long, plus their password. The online shopping website will have a low traffic, I don't think that more than 10 users will ever be logged in at the same time, probably even less than that, but there are 20000 possible users (max). Is there any drawback with this setup ? Should I use something like exUserFolder instead ? bye, and thanks in advance. Jerome Alet