Is there a limit to how often you can upload builds to TestFlight?

Yes.

We use Fastlane to automate TF uploading, and our build server went crazy and kept building and uploading over a weekend. At some point builds started failing with this error:

    [15:33:20]: [31m[Transporter Error Output]: ERROR ITMS-90382: "Upload limit reached. The upload limit for your application has been reached. Please wait 1 day and try again."
[0m
[15:33:20]: [31mTransporter transfer failed.[0m
[15:33:20]: [33m[0m
[15:33:20]: [31mERROR ITMS-90382: "Upload limit reached. The upload limit for your application has been reached. Please wait 1 day and try again."
[0m
[15:33:20]: [31mERROR ITMS-90382: "Upload limit reached. The upload limit for your application has been reached. Please wait 1 day and try again."
Return status of iTunes Transporter was 1: ERROR ITMS-90382: "Upload limit reached. The upload limit for your application has been reached. Please wait 1 day and try again.

It looks like we got banned after ~20 uploads, though I'm pretty sure we have had more over the course of a day. This was 20 uploads in less than 2 hours, so maybe that triggered it.

I don't think you're going to be able to hit this if you're an individual dev doing manual uploads. We've used Fastlane CI for a while and only had a problem when this recent problem happened.

Apple hasn't mentioned any such restriction in its documentation they say "As you continue to change your app, you can upload new builds"