I have 11 rentals and State Farm handles all the policies (300,000 business liability each) in a 1mm Commercial policy (no limit to # of rentals in it).
I used to use Markel Insurance as they also have a no limit for rentals for their umbrella. They were the most economical when my house was considered in a fire zone and I needed a non conforming type insurance co.
But now my primary home is no longer considered in a fire zone per State Farm, so by bundling my home, my vehicles, a personal umbrella and the commercial umbrella with them, believe it or not they came out the lowest and I did call quite a few others.
I talked to some roofers who advised me to go with State Farm as they are the fastest and best at replacements as well. I have used State Farm for underlying for a little under 20 yrs and they also came out with the lowest quotes. I have had to use them twice in natural catastrophes and had zero problems, fast service with good results.
I do not work for either of these places.