i have a routerboared with 4 ethernet lines ...and one port for users
the routerboard has 64 MB of memory
and i want to merge 4 line of adsl
the question here is.... how many maximum users can be add to this routerboard ??

Wow
Your question is not clear!! what do you mean by

how many maximum users can be added to a mikrotik??

What type of user do you have? PPPoE, PPTP or you managed bandwidth with add queue on static IP?
Tell more!!

i have an network of 35 users
and i want to increase this number of users to 100 user as an example
so i will buy a routerboard with RAM (64 MB)
can I merge 3 ADSL lines to reach the 100 users and can ( 64 MB only)be able to increase this nubmer of users or need a RAM (1 GB)

let be honest.
you need to serve 100 users, so what is your strategy for mange the bandwidth? It's important, because RAM and CPU usage depend on the rules you have in firewall, Queue, Routing and so on.
If you use V5.x , you can see in tool profile which shows you the usage of cpu