100% a personal decision :). For me, it's infinity. I've invested in both SFR's and MFR's (even a few 10+ units). My SFR's do a lot better than the MFR's I have. Of course, a ton of variables come into play here. Mainly where you invest. Where I invest, MFR's are fewer and further between so the cash flow on them isn't very attractive. For where I invest, SFR's are the play. We're actually moving away from MFR's and getting back to SFR's as it's better cash flow and value for our money. That's a really high level statement of course and there are a lot of factors to consider, but for me, SFR's is where I'm staying for the time being.
Also, 2-4 units fall into that "SFR" category for me. I know that's technically an MFR, but to clarify, we're staying with 1-4 unit properties and staying away from the bigger MFR's. I do have 1 5-unit that does pretty well, so the # is slightly flexible, but the 10+ apartment type units is what we're staying away from where we invest.