It’ll break up into multiple messages.
At connection you’ll generally get
CAP ACK's
001
002
003
004
375
372
376
JOIN
ROOMSTATE
353
353
As many 353's as needed
366
So you will get one or more 353 messages.
And then recieving a 366 will indicate that you have recieved all the NAMES.
I suspect that Twitch is compliant with the IRC RFC here, since this is to maintain some level of compliancy with regular IRC clients. So the max length should confirm to what the RFC suggests for max lenght of a single 353 message.
But I just ignore 353/366 since I utilise Get Chatters if I need chatters