" . . . we can do port forward for only one machine in that client network for those port numbers.The same port number can not be forwarded to more than one machine from a network."
Your IT-admin is basically correct. However you can change the port number your clients listen on and configure additional port forwarding rules to direct external traffic to multiple machines.
For example configure a client with a fixed internal IP address of 10.10.10.100 to listen on port 5901 and create the appropriate port forwarding rule. Move onto the next client with a fixed IP of 10.10.10.101 to listen on port 5902 and so-on. You use ARD to change the port number the client listens on. The firewall at your local network's edge to create the forwarding rules. However, depending on the numbers and hardware involved, this may become impractical because of the leg-work, manual configuration and hardware limitations involved.
Better to use a VPN.
" . . . Is it possible to add all machines from client network to ARD admin (which is connected to my Local network)? if Yes,
How can I add all client Mac machines(which are connected to client network) to ARD Admin (which is connected to my Local network)?"
Use a VPN.
VPN connections are encrypted and secure. It places the remote computer directly with a dynamically assigned local IP address (once the connection has been established) on that local network. It will then behave as if it was on that local network and not miles away. Essentially removing the need for multiple port forwarding rules (open to all sorts of attacks from port knockers and hackers etc) and manual client configuration.
This is what most network/IT administrators do as none of it is unknown or technically difficult. Therefore I'm a little surprised your IT-admin did not explain or suggest this to you as it should be well within his/her's capabilities as well as the hardware they're using at the local network's edge.
