[azpt]

If I purchase multiple fish at once from the LFS (say 6 damsels or something) can I freshwater dip them together, or do they need to be individually dipped one at a time?

[leebca]

If you're following my recommended procedure here:
Freshwater Dip for Marine Fishes, then you definitely want to dip one fish at a time.

Physical reasons:
A fish can easily harm another fish during the process and it would be a shame to have relatively clean and healthy fish only to have them injure each other during your dipping. Besides the obvious of fish running into fish, they have the ability to bite each other during the process which would be unwanted stress on both the biter and the one bitten. The presence of other fish can cause a fish to dart and thereby hurting itself. Lastly, too many fish means crowding -- a space stress.

Chemical reasons:
One fish will need all the oxygen it can get. No need for the competition taking up additional oxygen and releasing more wastes (carbon dioxide, ammonia, etc.) into the water.

Disease reasons:
If a parasite or microbe is on one of the fish and comes off during the treatment, it could manage to lodge itself onto another fish in the vicinity. Reinfection and trading parasites are reduced, although this reason is very 'weak.'