I’ve been painting my Goblins in batches of around 5. This is enough for the paint to dry so I can start on the first again after finishing the 5th. Maybe exepting the first layer and the ink layers. Arguably a full batch painting of 10-ish or even more is quicker, but painting in batches of 5 allows me to get a batch finished in an evening of painting, which motivates me far more than 20+ half-painted models on the painting table.
For my abyssals/ghouls I DID paint in batches of ~15 due to a very limited colour palette. (Basecoat, 2 colours, ink wash, highlight, details, done) which allowed me to finish them really quickly. I believe, I got two hordes worth of models (~50-ish) within a month and a half, including multibasing.
These days, I just slowly paint myself in little batches of 3-5 as I can take my time painting those models. My armies are big enough, extra models are just for brownie points.
EDIT: @FredOslow, I promised pictures of your soul reaver infantry idea that I stole. Here’s one: