I want to migrate Jira server into Data Center with 1 node only.
I understand that is recommended to have a load balancer, but what if it only has 1 node?
If you have only 1 node, then the load balancer isn't of much help. If it's not a load balancer, then what do you have plans to replace load balancer with (reverse proxy?). You can still use a load balancer just to send traffic to a single node and act like a reverse proxy or use nginx or Apache as a reverse proxy for your node. Both options work for a single node and it's up to you.