The ratelimit is 800 requests per minute, and each request to Get Streams supports 100 channels, so you could request 80,000 channels per minute, or if you have more channels than that you could poll every 5 minutes and get 400,000 channels.
Alternatively, you could use EventSub and on your server subscribe to the stream.online and stream.offline types to see when those channels go online and offline. If the channels you’re wishing to track haven’t authed to your app, you’ll be limited to 10,000 subscriptions though.