Assuming we have 4 ADSL from ISP, we can make PCC (Per Connection Classifier) Load Balancing as followings. You may adjust the number of WAN connections to suit your own needs.
/ip firewall mangle
add action=change-mss chain=forward comment=”” disabled=no new-mss=1480 protocol=tcp tcp-flags=syn
add action=mark-connection chain=input comment=”” disabled=no in-interface=pppoe-out1 new-connection-mark=pppoe-out1_conn passthrough=yes
add action=mark-connection chain=input comment=”” disabled=no in-interface=pppoe-out2 new-connection-mark=pppoe-out2_conn passthrough=yes
add action=mark-connection chain=input comment=”” disabled=no in-interface=pppoe-out3 new-connection-mark=pppoe-out3_conn passthrough=yes
add action=mark-connection chain=input comment=”” disabled=no in-interface=pppoe-out4 new-connection-mark=pppoe-out4_conn passthrough=yes
add action=mark-routing chain=output comment=”” connection-mark=pppoe-out1_conn disabled=no new-routing-mark=to_pppoe-out1 passthrough=yes
add action=mark-routing chain=output comment=”” connection-mark=pppoe-out2_conn disabled=no new-routing-mark=to_pppoe-out2 passthrough=yes
add action=mark-routing chain=output comment=”” connection-mark=pppoe-out3_conn disabled=no new-routing-mark=to_pppoe-out3 passthrough=yes
add action=mark-routing chain=output comment=”” connection-mark=pppoe-out4_conn disabled=no new-routing-mark=to_pppoe-out4 passthrough=yes
add action=mark-connection chain=prerouting comment=”” disabled=no dst-address-type=!local new-connection-mark=pppoe-out1_conn passthrough=yes per-connection-classifier=both-addresses:4/0 src-address=192.168.0.0/24
add action=mark-connection chain=prerouting comment=”” disabled=no dst-address-type=!local new-connection-mark=pppoe-out2_conn passthrough=yes per-connection-classifier=both-addresses:4/1 src-address=192.168.0.0/24
add action=mark-connection chain=prerouting comment=”” disabled=no dst-address-type=!local new-connection-mark=pppoe-out3_conn passthrough=yes per-connection-classifier=both-addresses:4/2 src-address=192.168.0.0/24
add action=mark-connection chain=prerouting comment=”” disabled=no dst-address-type=!local new-connection-mark=pppoe-out4_conn passthrough=yes per-connection-classifier=both-addresses:4/3 src-address=192.168.0.0/24
add action=mark-routing chain=prerouting comment=”” connection-mark=pppoe-out1_conn disabled=no new-routing-mark=to_pppoe-out1 passthrough=yes src-address=192.168.0.0/24
add action=mark-routing chain=prerouting comment=”” connection-mark=pppoe-out2_conn disabled=no new-routing-mark=to_pppoe-out2 passthrough=yes src-address=192.168.0.0/24
add action=mark-routing chain=prerouting comment=”” connection-mark=pppoe-out3_conn disabled=no new-routing-mark=to_pppoe-out3 passthrough=yes src-address=192.168.0.0/24
add action=mark-routing chain=prerouting comment=”” connection-mark=pppoe-out4_conn disabled=no new-routing-mark=to_pppoe-out4 passthrough=yes src-address=192.168.0.0/24
/ip route
add comment=adsl1 disabled=no distance=1 dst-address=0.0.0.0/0 gateway=pppoe-out1 routing-mark=to_pppoe-out1
add comment=adsl2 disabled=no distance=1 dst-address=0.0.0.0/0 gateway=pppoe-out2 routing-mark=to_pppoe-out2
add comment=adsl3 disabled=no distance=1 dst-address=0.0.0.0/0 gateway=pppoe-out3 routing-mark=to_pppoe-out3
add comment=adsl4 disabled=no distance=1 dst-address=0.0.0.0/0 gateway=pppoe-out4 routing-mark=to_pppoe-out4
add comment=adsl1 disabled=no distance=1 dst-address=0.0.0.0/0 gateway=pppoe-out1
add comment=adsl2 disabled=no distance=2 dst-address=0.0.0.0/0 gateway=pppoe-out2
add comment=adsl3 disabled=no distance=3 dst-address=0.0.0.0/0 gateway=pppoe-out3
add comment=adsl4 disabled=no distance=4 dst-address=0.0.0.0/0 gateway=pppoe-out4
add check-gateway=ping comment=adsl1 disabled=no distance=10 dst-address=0.0.0.0/0 gateway=pppoe-out1
add check-gateway=ping comment=adsl2 disabled=no distance=10 dst-address=0.0.0.0/0 gateway=pppoe-out2
add check-gateway=ping comment=adsl3 disabled=no distance=10 dst-address=0.0.0.0/0 gateway=pppoe-out3
add check-gateway=ping comment=adsl4 disabled=no distance=10 dst-address=0.0.0.0/0 gateway=pppoe-out4
/ip firewall nat
add action=masquerade chain=srcnat comment=”” disabled=no out-interface=pppoe-out1
add action=masquerade chain=srcnat comment=”” disabled=no out-interface=pppoe-out2
add action=masquerade chain=srcnat comment=”” disabled=no out-interface=pppoe-out3
add action=masquerade chain=srcnat comment=”” disabled=no out-interface=pppoe-out4