Well, it would best to compare apples to apples, there are 8M total Mastodon accounts to 108M Threads accounts.

For active users if we assume the same active ratio (probably not, but I couldn't find any data for Threads on that from a 1 minute search), then it would be 2 million to 27 million, which is small but it wouldn't "completely" drown out Mastodon.