It's not quite as simple as that, unfortunately.
The biggest factor for determining the park's maximum capacity is actually not related to the rides at all, it's more to do with the 'safety' aspect of things. My experience is mostly around buildings, but you can extrapolate outwards from there. There will be differences (outdoor spaces are covered by different regulations), but the principles are broadly the same.
- How many fire exits are there? In an emergency, you wouldn't want to have everyone trying to squeeze out of a single narrow exit point (partly as 'what happens if the exit is blocked', but also to avoid crushes, etc). Either reduce the theoretical maximum capacity, or provide more exits. Worth pointing out that for buildings, this "maximum fire occupancy" is often significantly higher than you would ever truly expect (factors of safety, etc).
- How many entrances are there for emergency services? If you can only get an ambulance in one entrance three miles away from the end of the park, you'd better reduce the risk of an accident. Or provide more entrances? Nice if you can.
- How many facilities can the park actually support? There are regulations that govern how many toilets you need to provide for a given number of people, and by extension you can work out water and drainage you need. Some sites will have restrictions on this!
- How much land area do you have for all those people? No good having a park surrounded by fire exits, tons of loos, etc, if more than 2000 people would turn it into a scrum.
- How much parking do you have? No good having a park that can handle 10,000 people if you've only got parking for 1000 (and, let's say, less than decent public transport).
- Will the local public infrastructure (roads or buses or trams or whatever) be able to cope with your park kicking out all guests at closing time without causing total gridlock?
- I bet I've missed dozens of other factors...
So the specific capacities for specific parks are usually something you can calculate 'for fun'. They're agreed with the authorities, insurers, operations teams, everything. Parks will, generally, have most of the boxes ticked, so I would suggest the simplest thing to do would be to get as many capacity figures you can from different parks and work out what that equates to as a 'people per square meter'. You may find (ignoring some weird outliers like Alton or Grona) that the number works out to something roughly average.
Other than that, I wouldn't get hung up in the detail.