It depends on leverage, property quality, location and size of town.
A well maintained A asset in an awesome high demand location in a big city with a 50% mortgage needs almost no reserve as even in a downturn you will find tenants and you will be OK even if rents have to be lowered 20%.
A poorly maintained C asset with a leaky roof in a small town with one employer in a poor location with an 80% mortgage will need far far higher reserves.
3-6 months of mortgage payments is a good rule of thumb, via an LOC, but it could be lower or may be higher.