You can increase the unit and building limit, for Single player only, in your options file.

The file is located at \Documents\Stronghold Warlords\Profiles\options.xml
There are values for 'single_player_building_limit' and 'single_player_troop_limit'
Please be aware raising these values will affect performance.

In single player Custom Skirmish, there is also an option in the Advanced Options menu to set the maximum troop limit with a list of 4 options.