Maximum number of outbound full relay connections What is the maximum number of outbound full relay connections a bitcoin core full node can make?
I was assuming it's 8 but then realized we can add another 8 connections using addnode while reviewing this PR: https://github.com/bitcoin/bitcoin/pull/21710
So is it 16 or there is something else that I am missing which can be used to add more outbound full relay connections?
Also do we have any option to decrease the number of MAX_OUTBOUND_FULL_RELAY_CONNECTIONS below 8?
What is the maximum number of outbound full relay connections a bitcoin core full node can make?

Maximum number of outbound full relay connections is 16(8 automatic and 8 manual)
MAX_OUTBOUND_FULL_RELAY_CONNECTIONS 8 (automatic)
MAX_ADDNODE_CONNECTIONS 8 (manual)
If you add 8 trusted nodes with -connect option, those will be the only nodes used for outbound full relay connections and nothing else.

Also do we have any option to decrease the number of MAX_OUTBOUND_FULL_RELAY_CONNECTIONS below 8?

No. And the developers who reviewed PR: https://github.com/bitcoin/bitcoin/pull/21815  think we don't need such option in Bitcoin Core.