Hi,

i tested 10 client under a public ip let say 10.10.10.2/32,
when the first client connected to remote pptp server, the second client under the LAN of 10.10.10.2 is not allowed to connect the pptp server.

just to clarify, is pptp only allowed 1 public ip ???

If you mean can you receive and process a PPTP connection request from multiple clients masqueraded behind one remote public IP then yes that should work as each connection will have a unique src-port.

For example 10 of your staff are staying in a hotel and all want to connect back to the office via PPTP at the same time - that should work ok.

You must have your PPTP setup to allow each user to receive a unique IP address, or if they all use the same account, select the IP from a pool and ensure multiple logons are enabled in the profile.

No it probably will not, because GRE, which is used for the actual data transport, doesn't know anything about ports.