How many do you suggest for a 300 gallon with quite a few Aiptasia
Since you have such a big tank, what I actually recommend you do is buy around 10 of them. In a side container (you can use a breeder net), put at least two of them aside. Throw the rest of them in your display.
The ones you set aside will be your breeders. You will have a constant supply of them so long as you can keep them fed. If you buy relatively mature nudis, they will lay eggs at least once a week -- usually more. As long as you make sure the breeder container always has aiptasia in it, you will soon find a steady supply of new slugs. This is useful because:
1) Any missed aiptasia -- and there could be lots considering the size of your tank and relative stupidity of the nudibranch -- will start spreading again and it's nice to have replacements (the ones in the display will die once they cannot find more aiptasia)
2) Fish might eat them. Though, I have not seen this happen in my aquarium and I have a vlamingi and a labouti wrasse (I have seen vlamingi TRY to eat one, but he spit it out immediately and it lived). In any case, it won't matter if you have more :)
3) You can SELL them :)
I actually did exactly the above (re: #3, local only) and, as I'm sure you'll find if you discuss this w/ others who have done similar, the hardest part is actually getting enough aiptasia to keep them fed :)
Good luck!
p.s. If you don't want to go that route, I'd say probably around 30 would suffice. Alternatively, just throw all 10 in the display. As long as nothing eats them (or their eggs), they'll reproduce in tank. It bears mentioning that amphipods WILL eat the eggs. You'll find people talking about this in the forums and I corroborate their findings.