Bad move by Facebook, but understandable given the heat they’re under.
TLDR: No more reach estimates on custom audiences and limits on how many custom audiences you can create. I don’t know the limit.
// Official announcement (ht Massimo Chieruzzi):
Facebook is committed to protecting the integrity of our platform and building for people first. After identifying a new technical issue with reach estimation for Custom Audiences that could potentially allow misuse of the functionality, Facebook is removing the ability to see reach estimates for all Custom Audiences across all of our interfaces to fix it.
Given the identification of this issue, reach estimates for all types of Custom Audiences will no longer be available in the following tools:
<list of all tools>
You will still be able to create or edit ads using any type of Custom Audiences on Facebook. To prevent misuse of our systems, there will be a limit to the amount of audiences created or edited in a day. You’ll be notified if you hit your daily limit, but will still be able to make changes the next day. Ads delivery and reporting will remain unaffected by this change.