The limit is how many messages your client can handle before the message buffer fills, at which point Twitch will just kill the connection.
So if you’re in some of the most active channels on Twitch then you’ll be able to join fewer channels per connection than if most of the channels were idle, or active at different times of day.
If you want to really get the most out of each connection you could track usage and balance channels over connections but that’s really overkill, most devs just use limits between 50 to 200 channels per connection depending on the specific needs/capabilities of the bot.