Supermetrics for Data Studio licenses have a restriction for the maximum number of Ad Accounts you have have set on Ad Network connectors, based on your license. The current limits are defined as:
Hobby - (3)
Individual - (3)
Pro - (20)
Super Pro - (100)
(See Also: Can I purchase more accounts for my license?)
These limits only apply to these Ad Network connectors and the accounts you select for them:
Facebook Ads, Twitter Ads, Bing Ads, LinkedIn Ads, AdWords, Yahoo Gemini, DoubleClick, Adform
And the limit is per Ad Network - so for example if you had a Pro-level license, you could select up to 20 Facebook Ads accounts, 20 Bing Ads account, 20 AdWords accounts, and so on.
Please note that these restrictions do not apply to Supermetrics for Google Sheets.