I looked into reinforcements, and it's set to give 2 waves on a normal battle and 7 on a siege.
This can be changed by opening up mission_templates.txt and finding:
2147483678 2 144115188075856126 #
(search for it without the #, I just put that there to explain). That's a check to see if defender reinforcements is below the #. Every reinforcement wave adds 1 to the number, so it sends # reinforcement waves. There should be 4 instances of it: Lead charge, village raid, castle with belfry, and castle with ladder. On lead charge and village raid it's set to 2, while on the castles it's set to 7. Changing those numbers will increase or decrease the number of reinforcement waves available to the defender.
The attacker reinforcements are under
2147483678 2 144115188075856127 #
You'll find 4 instances of this too, all right below the defender reinforcement ones. The numbers here are default 2 for lead charge and village raid, and default 5 for both castles. Changing these will make the attacker get more or less reinforcement waves.
This SHOULD work, but I haven't had time to test it just yet - and I have class in a couple hours so I won't be able to until later.