Can I use a single call rule set with multiple rules in place of multiple call rule sets?

It depends on how much you want to share across campaigns. The only reason to break up rules into different rule sets is if you don’t plan on using them all in the same campaigns. Example scenario: You created RuleA, RuleB, RuleC, and RuleD.  You want to use RuleA and RuleB in all campaigns. You want to use RuleC only in CampaignA and RuleD only in CampaignB. In this case, it would make sense to create three call rule sets: One with RuleA and RuleB, one with RuleC, and one with RuleD.