A general introduction is provided in Rule body and rule head and Rule filtering.
To control the number of rules you can define an upper limit for the length of the rules including both, the rule head and the rule body, that is, the maximum number of items in a rule.
To specify maximum rule length set the maximum rule length in the DM_RuleSettings value by using the method DM_setMaxLen. Retrieve this value by using DM_getMaxLen.