If I bought the license for 25 users, do I meet the problems with adding new 25 users in the future??

[Nic Brough -Adaptavist-]

When you try to add user 26, it will stop you if you are using internal user lists. If external, some of the functions will stop working (adding issues is the main one).

You will either have to get a 50 user licence, or disable users until the count is down to 25 active users.