Gary,
The general 'generic' answer is as many as you can stand to
put in, and as long as you can make them. Usually, the length is a 1/4
wave (or sort of close) for the lowest frequency of use. Don't have
that much room? Then make them as long as possible. The more you put
down the 'better' it'll be. That's where the 'as many as you can stand'
comes in. At some point you'll be ready to quit, so quit...
'Doc

PS - The 'text book' answer? Not really, but probably the average one.