Space for Camping

Hi everyone!

We are planning an event (hopefully) for next autumn. All first time event organisers and therefore need a bit of advice please!

How much room do you legally have to allow on a campsite? Is it per person presumably as you have no way to know how many tents or people per tent.
Basically we have an amount of space on the site but I need to somehow work out how many campers we can fit into that space!
Obviously I know I will need to allow space for toilets and water points etc but no idea how you calculate capacity?

Thanks!