I have had assam spawn. I started with 9 fish, they made 3 pairs and 2 of the pairs spawned. I would think 9 would be minimum. I tried to get pairs out of 6 assams and I didn't get pairs. When I was breeding gachuas I would get better pairs and more pairs if the group was larger, 9-20 fish. I have also bread rainbows I started with 15 fish they made 3 pairs and only one pair ever spawned.
Start off with as many fish as you can. You could put as many as 20 assams in your tank, they are small.