Note: The relevant question in WordPress.com's case is not the number of IP addresses but the number of certificates per registered domain. See Rate limits for Let's Encrypt for details of how the limits work. Also check out the Integration Guide. I think in your case you will probably not need any overrides, assuming you use a single account.
Also, to be clear: No one is exempt for rate limits, but we do have an ad-hoc method to increase rate limits on a per-account or per-registered domain basis. We're still figuring out how to scale that process, as @pfg said.