We picked up a set of antique bunk beds for my son yesterday. They do not have all the slats - only 3 slats were included. How many do we need, to hold up a twin mattress?
My boys’ bed has 12 for each mattress.
Note that if there’s an IKEA near you, you can buy bed slats there. I have a set I use on a guest bed.
Is this like one of those word problems? Math is hard. 
Let’s go shopping!
Dave picked up a plank and cut it down, we will try it with up to 8. We’ll just set up one bed for now. $3.00 for a plank, and how much for the slats at a store? IKEA was actually my first thought - but thanks for your suggestions!
Gingy, I went and looked but ours are custom made and require no slats. The slats under my son’s full size bed, however, are four in number. These I made in a snit one night with a chain saw and a couple of 2 x 4’s. 
Just remember that you need MORE slats up the top end of the bed than down the bottom to accomodate the greater body weight.
Aside from that, I would have at least 8 on your average bed…more depending on the mattress.
Heh. No snit involved here - and they’re 1x4 rather than 2x4. Total was $8.94 with some leftover pieces.