How many spaces on average does track day have?

[TAA5]

Does it change a lot depending on the circuit?

[Pericoloso]

Open pitlane is likely to have lower limit than sessions.

Yes ,it will depend on circuit's own number restrictions.

[sfaulds]

While the number of cars on track at any time is governed by the circuit, the number of attendees is down to the organiser. As a general rule of thumb, the cheaper the day, the more attendees they will allow.