It's tricky to give a number for "RA required" that isn't wildly misleading, or contextualize one you're given. The common case for most AV RAs is confirmation of what the vehicle already has planned. Does that count as "required"?

An AV company can also tune how proactive vehicles are in reaching out to RA for confirmation, which is a balancing act between incident rate, stoppages, RA availability, and rider metrics. There's other ways to tune RA rate by also adjusting when and where the vehicles operate, which comes down to standard taxi fleet management tools (e.g. price and availability).

Waymo chooses a target that they're comfortable with and probably changes it every so often, but those numbers aren't the only possible targets and they're not necessarily well-correlated to the system's "true" capabilities (which are themselves difficult to understand).