I'm assuming you're playing on very hard (if not, don't complain about too low difficulty
)
Find this section in the campaign script:
Code:
monitor_event PreFactionTurnStart FactionIsLocal
if not I_EventCounter DifficultyLevel = 4 ;very hard
terminate_monitor
end_if
if I_CompareCounter mongol_spawning = 5
set_counter mongol_spawning 0
end_if
inc_counter mongol_spawning 1
end_monitor
Then just change "if I_CompareCounter mongol_spawning = 5" to a lower number. The number says how often, in turns, the Mongols get new reinforcements.