The audience size information is currently withheld by Facebook for certain audience types. You may find your Custom Audiences show "-1" or "Not available" or "20". All of these just mean that Facebook is not allowing you to see the actual audience size. As long as you see the status of "Ready" these audiences are still good to go... Please scroll down to watch the video for a workaround to figure out the actual sizes of your Custom Audiences! Here is the official message from.

From FB Support: 

"We identified a technical issue with reach estimation for Custom Audiences that could potentially allow misuse of the tool
We're temporarily removing the ability to see reach estimates via the API for the following Custom Audience types:
• Custom Audiences from a customer data file.
• Offline conversions Custom Audiences.
• Website Custom Audiences that use Advanced Matching.
• Mobile App Custom Audiences that use Advanced Matching.
• You will still be able to create new Custom Audiences, and use them for ads targeting.
• Ads delivery and reporting will remain unaffected by this change.
• We are currently investigating solutions to restore these reach estimates
So essentially we found that there could be errors with the Audience amount, because of this we have disabled that for the moment while we look into what could be the cause."

According to FB these audiences are "Ready" to use and work both for direct use and use as base audiences for Lookalike audiences.

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