How do I increase the number of connections?

I know that TCP connection Default is 30000.
How do I change it?

Couchbase Server version 6.0

You can tune your TCP settings however you must be careful to not encroach on well known ports in use - i.e. those in use for Couchbase Server. So typically I would advise you don’t lower the ephemeral (or local) port range below 24000 - and this should be verified as “safe” (must not result in a range that overlaps with listening software) for your specific system and the software running on it.

You can read about “IP Variables” and how to set ip_local_port_range here: IP Sysctl — The Linux Kernel documentation