HI webchun
Seblod is a great tool exactly for these kinds o things. It would, however, require some PHP/MySQL skills.
1) The Session content type should have a participant counter filed (INT column in the content type DB table)
2) I assume you have a Select Dynamic Filed in your registration form for selecting the session. You have to update the SQL quoery of the field so it offers only those sessions where the participant counter column is < 100.
3) Use the SEBLOD after sore plugin to increment the participant counter column of the selected session after each user registration.
Hope this helps
Michal