Ok, I'm a newbie dipe sewer too...but I've sewn up a lot of doublers in the past few days (my lil man is a heavy wetter
I made mini-doublers that were 5in long by 3in wide. I had a yard of hemp fleece and I got about ten 3 ply mini-doublers out of it.
The number of inserts you're going to get will vary by the size and the number of layers in your doublers/inserts. I tried the two ply inserts before and they were just too thin. Four ply inserts are pretty thick if you're using fleece...you may want to make up a couple of test inserts and see if they're too bulky.
BTW I made mini-doublers to go on top of my regular JB inserts when I stuff my FB and HH. Putting two JBs in the pocket was too bulky for my taste so I put in the extra layers where I needed it...up front since my babe is a boy.
Oh, I also don't have a serger so I used the overcasting stich on my machine instead of turning and topstitching. The zigzag stitch works good too.