@Idris59
Why not just charge on a per person basis? Set a base price for up to x-persons. After that, they pay say, 20 per person per night. It's all there in your listing setup.
Having said that, it's quite common these days that some guests will book for 2 and bring 6 or 8 (it's much cheaper that way). Then act surprised that there's an additional charge for more persons (if they get caught).
Unfortunately, if you have these types of guests, there's very little you can do without suffering some sort of penalty. If you do manage to catch them and try to charge them or make a complaint with Airbnb, it's likely you'll get a retaliatory review complete with fabricated atrocities you never committed against the guest, which will cause significant damage to your ratings. The guests may even file a claim with Airbnb for those imaginary atrocities and quite possibly get a partial or full refund from Airbnb. So, you need to tread very carefully with dubious guests.
That's why it's wise to ask questions before they book, in an attempt to sniff out what their real intentions are. If it smells funny, it usually is.
Good luck and happy hosting
