what would happen if usage gets 100 percent. No user will be able to login and all logged users will lost terminals?
Generally, yes, if
/home fills up the system will become largely unusable. People will probably not lose their existing terminal sessions, but they will be effectively dead in the water since they will not be able to create any more files.
New logins will generally fail because many tools want to write data into your home directory as part of the login process. Simple command-line logins (e.g., using ssh) may succeed but the system won't be particularly useful.
If I understand this right every one user can full up the /home and that will affect all users.
This is why many storage administrators implement filesystem quotas...to prevent users from filling up a shared filesystem. If you have 150 users and only 6GB of space, you want to make sure that no single user can use more than 40MB of space if you want to evenly distribute disk space.
You may also want to purchase additional disk space and expand the filesystem, because it sounds like you're oversubscribed given the number of users and the available space.
Here is an article that talks about using quotas in Linux.