Depends on your panel. Assuming they are right next to each other, with minimal gap, no structural members to traverse, no other cabling in the way, you could probably get away with six. But you're the only person who can make that determination. Also remember you'll want at least some slack for sliding boxes in and out of the panel. Without any slack, you'll have to reach behind the panel and disconnect them before removing the units. That works fine with some panels, but not so well with others. Again, that's something only you can determine. If in doubt, I'd probably opt for the eight. There's very little chance that extra two inches of cable will cause you problems.