Roughly cap the number of undead spawns to avoid performance issues

data/campaigns/The_South_Guard/scenarios/06b_The_Long_March.cfg

@@ -1040,6 +1040,13 @@ _f, _f, Re, _f
                 greater_than=$first_undead_spawn_turn
             [/variable]
 
+            # Don't spawn more than is really needed, for performance reasons
+            [have_unit]
+                side=2
+                race=bats,undead
+                count=0-50
+            [/have_unit]
+
             [then]
                 {VARIABLE max_undead_spawns $turn_number}
                 {VARIABLE_OP max_undead_spawns sub 2}