Google supports 10,000 campaign references to upgraded ad extensions per account & 100,000 active upgraded ad extensions per account.
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I agree with Shuva . But as you have asked you can select only 20 active site link at a time per campaign or adgroup. You will see following notification when you will exceed limit of 20:
"You selected too many sitelinks. Choose up to 20."
Hope that helps
Re: Sitelinks Extensions Limit in AdWords Editor
I think there are 2 issues here :
1) How many sitelinks can we have in the left panel , let's call it "storage" as they are not active , this panel just gives us the choice to select them to be active at the campaign or adgroup level
2) How many sitelinks can be active per campaign or ad group in the panel in the right side, and yes these active sitelinks can be at most 20.