Over the years there's been a fair amount of debate on this topic. On one hand, you have people who just throw every keyword they have in 1 ad group. Then on the other hand, you have people who will only put a single keyword per ad group (SKAG).
Which set up is most optimal for your Google Ad Campaign?
I think both scenarios outlined above are extreme. First off, you definitely don't want to put all of your keywords in 1 ad group because then everyone searching any of those keywords will be shown the same ad.
Single keyword ad groups (SKAGs) may be useful still in some scenarios but for the most part it's an outdated strategy.
In my opinion and experience, arbitrarily limiting the number of keywords that go in 1 ad group just leads to an unnecessary increase in workload.
Instead, you should just separate keywords into different 'themed' ad groups, so you can create 'themed' ads that are more relevant for the keywords in that ad group.
If that means you end up having 30+ keywords in the ad group, so be it. So long as they all share a similar theme and therefore are served the 'themed' ad to go with it.
For example, let's say you're a dentist and you offer general dentistry services and you also offer cosmetic dentistry. Each of these would have their own ad group, with their own set of keywords and a relevant ad to go with it.
You can even take it a step further and put each in their own campaign, which would give you more control over the budget & ad spend for each but this isn't always necessary.
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