Yes indeed this error may be the case of having to many VMs in a datastore. I think this may be a good point to re-evaluate your storage snapshot strategy, as it may be that only a percentage (say 20% of the 220) only really require consistent snapshots, whereas the rest may be perfectly fine with non-consistent snaps.
By doing the above it will help you create two levels of datastore, which are part of two separate Volume Collections (one which has syncronised snapshots, one which doesnt). Therefore you wouldn't neccessarily need to follow the best practice of 10-12 VMs per datastore, as the majority would be happy doing what you're doing today.
Also the "10-12 VMs Per Datastore" comment isn't just for snapshots, it's an overall Best Practice for ultimate performance & latency. Here's a great blog post by Jason Boche who really went into detail about why this is: VAAI and the Unlimited VMs per Datastore Urban Myth » boche.net – VMware vEvangelist