Does anyone know of ways to check the seat capacity or number of tickets sold or number of passengers onboard for past flights? I'm actually just trying to figure out how many passengers were on a specific flight, not the names of the passengers, just the number of passengers.
Unfortunately this is key competitive information and it's closely guarded by the airlines. Sorry.
If you just need a rough guess, you can look up past flights on a site like Flightradar24 and then check the capacity of the airplane in that airline's config with SeatGuru, which will give you an upper bound. Then check historical price data with Google Flights: on days/flights that are expensive, load factor will be close to 100%, while on cheaper ones it will be less (maybe 80%, rarely less these days).
However, this assumes normal flight loads. In 2020 due to COVID all bets are off and many airlines are flying with much lower load factors than usual.