On my backend instance a service is running which has to accept multiple connection in a second but TCP LB is not allowing multiple connection at a time.
Please help me to increase LB connection to max.
Where did you get the information that it only allows one connection at a time?
The Network Load Balancer (also known as TCP load balancer) allows you to balance the load of your systems based on incoming IP protocol data, such as address, port, and protocol type. As long as your instance services have the resources to handle the request, the load balancer will redirect the traffic.
You can read more about it from Google from this official document.