I feel that... I don't really know what size they're going to be, but you can probably get away with drawing 3-6 different people, then flipping them horizontally so you've got 6-12. Make some assorted groupings and color them differently. Copy-paste those. You could make the ones in the back black silhouettes like you did with gerbils potentially even - it might not look weird? Hard to say without seeing. And depends on scale how obvious this will be.
Honestly there's such a spectrum from "draw every person" to "bare minimum representation of humans" so that the reader knows it's a crowd.
In my case I did some very lazy blurring with all kinds of lighting/color on top to deal with my crowds for the most part, which wasn't super and wouldn't mesh with your art style anyway I think :(. Sorry I don't have any better ideas! Looking at some references on Pinterest that are similar to your art style might be a good place to start.