How to estimate repairs on a property?

[Landon Belk]

I'm trying to make a game plan to purchase anything from a duplex to a fourplex in order to house-hack it while making a little extra. I don't think taxes, insurance, or vacancy rates should be hard to estimate, but how do I estimate for possible repairs? How much do I need to be taking from the cash flow in order to prepare for unexpected repairs and maintenance? Is there a magic number for each unit? Do I need to account for age of the building? What about capex?