The limits are 300,000 static rules [1] + 30,000 dynamic rules [2] + 5,000 session rules [3]. For reference easylist is about 35k rules. The Chrome team has been constantly tweaking these limits themselves and Brave could set their own limits if they wish. The API is designed such that extensions can query to see how many rules they can use.
Note that "dynamic rules" as specified here are not the same as what GP was talking about. The API does not support dynamic rules in the way that Firefox/V2 users use that term.