Boveda says for every 25 sticks you use one pack plus one pack.

25 would be 2 (1 for the first 25, +1 additional pack)
50 would be 3 (1 for each 25, or 2, +1 additional pack)
75 would be 4 (1 for each 25, or 3, +1 additional pack)

I've used this method with my 50 count Cigar Caddy and it works great, one each at the top, middle, and bottom.