I found that Substrate has a limit of 64 pallets that can be added to the runtime. Adding more causes the following (very unobvious) error:
error[E0599]: the function or associated item `execute_block` exists for struct `Executive<Runtime, Block<Header<u32, BlakeTwo256>, ...>, ..., ..., ..., ...>`, but its trait bounds were not satisfied
I'm using this runtime as a template.
Is there a way to extend this limitation?