There are some A numbers below 20 that sometimes are not utilized, but otherwise, yes, you'd be correct, you're boarding number is indicative of how many people will board before you.
As for sitting together, there are around 20 - 23 rows on the plane, each with 3 seats on each side, so in theory, each row could have a person seated in one seat on each side, and you'd have to break up across aisles. Now realistically, this probably won't happen as there will be other families sitting together too (not SW isn't as big for business travel - so you don't have as many single business travelers). You should have no issues sitting together.