There's no fixed number, as you have too many other variables:
- How big are the people eating? (athletes are going to eat more than kids or people on diets)
- What are they being served with? (If it's just a table filled w/ crabs, they're going to eat more than if you have other side dishes available)
- How experienced are the people with eating crabs? (Some folks are so slow at picking crabs they'll give up in annoyance. Others are so skilled they'll wolf them down quickly. In high school, some folks I know from the Eastern Shore would pick the backfin out then trash the rest, ignoring the flake and the claws)
- How are they being served? (very spicy will slow most people down. And you have to remind the crab newbies to wash their hands before using the bathroom)
- What temperature is it? (on really hot days, people eat less)
- How long is it going to be? (if you're hanging out all day, people might get a second wind and go for more crabs)
I'm going to guess that if they're 'mixed', they're probably mediums, so maybe 7 or 8 dozen per bushell. 3 or 4 crabs per person is about typical, adjusting mostly based on the the factors above. (large, physically active folks or experienced pickers might eat 6 or more ... inexperienced folks often give up by the second one ... but you'll want more if you're doing it as an all-day thing, or if it's almost all crabs and hardly any sides)
I guess just make sure you have plenty of drinks and some sides for those that give up on the crabs or if you run out.
And don't just go and pitch the shells afterwards ... you can generally get quite a bit more meat going them a second look through (the folks who skipped claws if nothing else), and picking whatever's left over. Freeze that meat, then rinse the legs (most people don't crack and suck them) to get most of the seasoning off, crack them, and simmer them to make a crab stock for bisque. If you have a really large pot, go for the rest of the shells, too ... strain, boil it down to concentrate it, then freeze.
You'd be tempted to add extra stuff to the stock besides the shells, but as we don't know how much seasoning's still on the crabs, you really need to wait 'til it's done, then add other stuff when using it to make the final dish.