We have 5 on the roster now; if we get two more commits, we have 7, plus Realus. 8 RB's is enough.
I wouldn't even count Burns as I doubt he'll ever be healthy enough to play. And it's possible that Homer will be gone after next year.
Two RBs for '19 is good, but I'd add a third if it's for an elite guy (obviously Sanders, but probably another guy or two).