I recently was asked the following questions about biscuit joinery:
"I'm using a biscuit joiner to glue up boards for a table top. What size biscuits should I use? How far apart should I space the biscuits?"
When using biscuits, you should always use the largest biscuits you can. In most cases, you'll use a #20 biscuit, but if this is too large, you can try a #10 or even #0 biscuit (for the smallest joints).
Your biscuit joiner should be adjustable to accommodate all three common biscuit sizes.
As far as how far apart to space the biscuits, this is really up to you, as long as you follow some basic guidelines. The biscuits on the edges of the joint should be no more than 2-3" from the edge of the joint. Once you've determined the positions for the edge biscuits, you can evenly space an appropriate number of biscuits between the two end biscuits, space them between anywhere 6-12" apart.