For context purposes, I own 5 SFRs, which I bought over the past two years.
I started with enough to "turn" a house over and get new tenants, plus 3 months of business expenses (taxes, insurance, debt payments, etc.) in cash or at least easily accessible. As time goes on, I added 15% of the rents to the fund for maintenance expenses, which builds up quickly. So when a house is vacant and I have to "turn" it, or when the roof gets a hole in it, I have enough cash to cover me.
You may want to adjust upwards if it takes you more than about a month to rent the home since you are 100% vacant when you are vacant at all. But I would do a little research and figure out what it costs to turn a property over (paint, flooring, cleaning, etc.) and go from there.
I think most folks would say that's way too conservative, but I am fairly risk adverse. My houses are cheap and small, and the rent comparably high. So 15% of rent is enough to cover maintenance, although I only have two years of data points.