Slowpoke, you mentioned the tree is 18-20" around. I assume you mean circumference. If that's the case, then the diameter should be about 6" and I would try to get two staves out of the tree......four staves at most. Try to split the tree so that the small limbs are on the edge of the stave or in the center at the handle area.....not in the main working part of the limbs. Hopefully, you can position the small limbs and knots so that you can remove them when you cut the tapers on the limbs.
On the other hand, if the tree is 18-20" in diameter then we have another whole ballgame. You can get at least a dozen staves or more. There will be a lot of options on how to split out staves from that tree.
Good luck and straight splitting.