Using 1.2.164 for 1.18.2, if I trigger tasks to pregen 4 dimensions (nether, end, overworld, twilight forest), it only pregens 2 at a time. It seems to only be pregenerating overworld and the end. If I cancel and only trigger pregen tasks for twilight forest and nether, it works fine.

Is there some limitation to how many dimensions can be pregenerated at the same time?

It also sometimes drops down to only pregenerating the overworld only, even though the other tasks are still there

There are no limitations built into chunky. It depends entirely on the support of the server platform you are running on. For example, on Paper all 3 vanilla dimensions can be generated simultaneously. On Fabric/Forge (without mods essentially vanilla) these kinds of restrictions have been observed. As far as I recall, this became more of an issue after Minecraft 1.18. There are effectively bottlenecks in the vanilla chunk gen system itself.

You'll have to figure out what works on your server. If you have any other questions, feel free to join the #chunky channel on our Discord server.