I can't get it to go above 90.

Try the Guides section of the forum for hints and spoilers.

There are exceptions to the 90 population limit like twins or triplets when there are 89 (or 88 with triplets) villagers, or an event that brings more to your village.

But the best way to increase your population is: to finish a collection. And when you finish all four collections, you will get another bonus.

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