The size of your audience is the number of visitors to Quora that match the criteria you set for that audience. It roughly represents the potential number of people your ads might reach if you target that audience.
An audience size of 0 is possible if there is no overlap between the people who visit your website and Quora's audience. For Website Traffic Audiences, a size of 0 may also indicate that your Base Pixel is inactive.
To view the size of your audience, visit the Audiences tab on your Ads Manager. Estimations will appear for Website Traffic and List Match Audiences. Sizes for Lookalike Audiences will be calculated under "Potential Weekly Impressions" when the audience is targeted in an ad set.