Ordinary 14 inch open top baskets usually support 3-4 Surfinias; which might give you a guide. Dead head regularly and can be cut back near the end of August for second flowering.

I've heard that with hanging baskets you cram the plants in. Not sure why this is though but I've always done it since.

by cramming them in they 'hide' the basket as they grow. also the congestion makes the plant go into 'flower before I die' mode. dead heading prolongs this biological need to set seed.

Glad I know the reason, Seaburngirl. Thanks

Been using Easy Fill baskets for a few years and always used all the holes the only thing is the large 12 hole baskets are very heavy make sure your brackets are well fixed I have had the chains supplied break and have replaced them with stronger solid chains.
http://www.growsonyou.com/photo/slideshow/298214-begonia-amber-shades/member/peterathome

One thing I forgot to say about the baskets is the very large ones need large brackets the largest I have found aren't really large enough and the plants at the rear tend to get battered so you may decide not to plant at the very back.
If they are the ones with the frets when planting I cover part of it with polythene because when watering find they drain through the frets to quickly it helps if you add water retaining gel when planting.