As documented in the Docs, PubSub only allows a total of 50 topic subscriptions per connection. You’re hitting this after 16 streamers (16*4 = 48). You can get ten times this limit by creating an additional PubSub connection, but i advise against this - PubSub is mainly intended for first Party use with third Party use as an afterthought - In contrast, I recommend you check out EventSub instead, as it has far greater limits (including being effectively unlimited if you have broadcaster authentication) and is intended for third party consumption from the ground up.
Note that it currently only supports a Webhook-based transport, but EventSub over WebSockets is coming soon™, as it is currently in closed beta.
Feel free to ask any questions about EventSub here or on Discord!