Answers
How many keywords should be in an ad group?
Few. Enough that every keyword in the group can share one honest ad.
The short answer
Few. Enough that every keyword in the group can share one honest ad.
The old advice of tightly themed groups still holds, for a simple reason: the ad has to match the search. Twenty unrelated keywords in one group means the ad matches none of them well.
With Smart Bidding, over-fragmenting hurts too — it splits conversion data so thin that nothing learns. Tight themes, not single-keyword groups.
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