I want to know how many users can I register in User Manager in Mikrotik 4.1 with L4 license,
I see there 20 sessions does it mean only 20 users I can register or only 20 users can check their state through web interface in the same time?

It means you can register as many users as you want, but only 20 sessions can be active at any time.
So if you limit each user to only 1 active session, then you can only have 20 users active at any time.
And if you allow each user an unlimited number of sessions, then it's possible to have only two active users each with 10 active sessions!
If you are a large ISP (or a small one but you envisage growth) and intend to deploy usermanager, go for a L6 license.

Again not clear to me. I tried and connected with my own PPPoE connection but at sessions does not shows any session? When I check at users I see that me time is counting down.

Does it means that at the same time 20 users can check their status in User Manager web page or generaly only 20 users can have internet connection in the same time if I have L4 license?