Michael, we do have a feature that can auto-delete rows, designed for high-volume webhooks. This is available only with the Enterprise plan (https://www.clay.com/pricing).
That said, when the limit is hit, Clay should return a 403 response, "Record limit reached". You can use that as signal to create/swap to a new webhook endpoint. So one option to consider is setting up multiple webhooks in advance, and then having your application cycle rotate through those endpoints (perhaps setting up a try/catch to retry and swap to another webhook when a limit is reached).