Mnaaah just use UPnP - start the UPnP service on the local interface on MT and enable UPnP on all 15 BitTorrent client applications. If UPnP is not supported try another client like uTorrent that supports it. NOT TESTED
or if want to do it the oldschool way:
You'll have to set up each puter to use a different outgoing port and write it down. Then on MT forward each port you wrote down to the puter thats using it. How to forward a port this way is described in the manual.
The server's address is now 192.168.0.4, and we are running web server on it that listens to the TCP
port 80. We want to make it accessible from the Internet at address:port 10.0.0.217:80. This can be
done by means of Static Network Address translation (NAT) at the MikroTik Router. The Public
address:port 10.0.0.217:80 will be translated to the Local address:port 192.168.0.4:80. One
destination NAT rule is required for translating the destination address and port:
[admin@MikroTik] ip firewall nat> add chain=dstnat action=dst-nat protocol=tcp
[admin@MikroTik] ip firewall nat> pr
Flags: X - disabled, I - invalid, D - dynamic
0 chain=dstnat dst-address=10.0.0.217/32 protocol=tcp dst-port=80
action=dst-nat to-addresses=192.168.0.4 to-ports=0-65535
Where you add as much as these rules as you need, change port 80 to whatever port you wrote down for each puter and IP address 192.168.0.4 to each IP of your 15 machines. 10.0.0.217->replace this with your gateway IP address or leave IP address fielt blank if you setup the rule for the public interface.... Consult the manual if confused.