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.
1984 Fast/Nicholson 345 "FastForward"
".. 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)