If you are to do this, and there are lots of people who would not, you want the $8,000 before any sexual contact with anyone, in cash, and be able to have somewhere to ensure that it is safe.
It is a very big if though - just what are you expected to do with twenty men over eight hours? Have them one at a time? (Ouch.) Be on the receiving end of an eight hour gang-bang? (OUCH!) How much rest will you have? etc etc. Whatever the answer, unless you have had a very interesting sex life, it will be unlike anything you have ever done before and no-one is going to say that it will be a good introduction to pay-for-play sex. It will be unforgettable, and very likely not in a good way.
Even if you are interested (and about the only scenario I can think of where that might be sensible is if you need $8,000 and only that amount, i.e. this is a complete one-off), if you cannot take along someone with your best interests at heart - and that's not your 'manager' - and who will ensure that your boundaries are respected, it's a big screaming sign to say 'No, why don't you do it?'
If you do it, get those boundaries - what you are prepared to do and what you are not - sorted in your mind, communicate them, and stick to them. Even if that means some of the delightful twenty don't get what they want. Especially if...