I believe Databricks series L round raised $4B in late 2025, but earlier this year they raised another $5B so technically they've maybe completed series M round and are "on" series N round now? The press releases are a bit confusing to me.

It's semantics, but the latest raise might have been a follow-on to Series M, not a new round (to be clear, I know nothing about their finances, just speaking from experience at another company).

What happens when you make it to Z do they then start doing Z1 rounds or does it reset to AA like excel (though that would be confusing)? Hex?

The AA scheme has already been used for "down round", so they might have to get creative.