User Log-Ins at the same time
I take it the video training is not what they are logging in to, but rather the CiviCRM system. There is no 'limit' but there will be server constraints. Depending on your server config, if you have 50 users and they all click on a complex query (such as an Include/Exclude Smart Group) then you may see smoke coming out of the screen ;-)

But fundamentally there is no limit to logged in users (which is a CMS issue rather than a CiviCRM issue anyway)