One plant or multiple plants in one pot does not really matter as far as growing good plants is concerned.
As James points out, with multiple plants in one pot, the flower show will be much better, and hence, many florists sell orchids in big display. better look and more cash.
As Tucker mentioned, if you ever wish to separate them, it can be a hell. lol
Another big possible issue is that if one of them is virused, the rest will all share the bug.