Create 10 address-lists that cover the 10 pools attached to the 10 PPTP server instances, then src-nat accordingly. Below an example with 2 PPTP servers, simply expand it:
PPTP server IP 1.1.1.2 serves the client IP pool 10.0.0.0/24
PPTP server IP 1.1.1.3 serves the client IP pool 10.0.1.0/24
/ip address-list
add list=pptp_server_1 address=10.0.0.0/24
add list=pptp_server_2 address=10.0.1.0/24
/ip firewall nat
add chain=srcnat out-interface=[name WAN interface] src-address-list=pptp_server_1 action=src-nat to-addresses=1.1.1.2
add chain=srcnat out-interface=[name WAN interface] src-address-list=pptp_server_2 action=src-nat to-addresses=1.1.1.3
You could do that without the address-lists and just specify 'src-address=10.0.0.0/24' and 'src-address=10.0.1.0/24' respectively, but if you ever attach more than one pool to a PPTP server it's easier to use address-lists, and it doesn't hurt to do it from the start.