After reading the well written tutorial on
https://community.openvpn.net/openvpn/wiki/EasyRSA3-OpenVPN-Howto, I have no problem generating and signing basic key pair for my own usage.
For learning purpose, to clarify my concept, if I sign 2 server-keys and 6 client-keys by a single CA root, will all 6 clients be able to connect to the 2 servers? If I want client A,B,C use server I, client D,E,F use server II; do I need 2 CA root each for one of the group? Or there are other option under the easyrsa command to handle this?