Hey Pauli,
This indeed sounds possible – you would simply need to buy and install both Events Calendar Pro and The Events Calendar: Community Events on your site. Then, let the 50 people register for your site and then turn off registrations so that other people cannot register.
As for your other questions about requiring specific fields, this unfortunately requires custom code 🙁
We cannot help with writing custom code, but we do have a knowledgebase article on this topic that should help get you started → https://theeventscalendar.com/knowledgebase/required-fields-for-events-submission-form/
Specifying character limits for each field would unfortunately require even more additional code 🙁
I hope this information is clear and helps you move forward with your project!
Cheers,
George