I would think a foot apart, staggered fore-and-aft with 2 per side would be plenty. Place only one exit at any given height in the mast.... but I'm not a rigger so it would be best to consult an expert.
On our boat they are closer together than that with no apparent issues.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"