I'm not sure I'd want to go past 10 STRs ever and I'm trying to decide if it's worth Co-Hosting or just save up for the next one.

After several calls with some of the larger PMS providers, where I really took the time to dive into their products and compare features, pricing, and more, we've decided to go with Hostfully.

For example how much to reserve for management fees (I don't have time to host myself), cleaning, electricity.  

The truth is, a good host will spend time managing each rental.

For California, here are a couple of options:California Vacation Rental InsuranceUnmatched Short-Term Rental Insurance for California Hosts 

I created a co-hosting profile as I was looking for additional opportunities.

We are working on more resources to help new and experienced hosts here at BiggerPockets.

As a super-host going back over 14 years, I know how important it is to have a great experience.

There is a good one in the Stamford area hosted by Brandon Chisholm.  

Bonus Tip: If this quadruplex is out of state, I’d highly recommend building a reliable local team (cleaners, handymen, etc.) and maybe consider hiring a co-host or VA to help with day-to-day tasks, especially if you scale up.