Bot API 429 errors in JMeter load test

Can anyone clarify the rate limits for the bot interaction endpoint?

Running JMeter 5.6 on US1.

Hit /api/v2/bots/{botId}/interactions.

Got 429 Too Many Requests at 15 threads.

Token is valid.

Need to know the exact threshold for capacity planning.

Docs are vague on concurrent bot session limits.