Marketing ads question

Hello!  I have a question regarding marketing ads and how many versions is the right number? Long story short, we are working with a new marketing agency this year. As we get ready to launch our spring/summer acquisition ads for our membership program, we have 4-5 different marketing ads that we will rotate over the next 19 weeks. Each ad concentrates on a different component of our membership program; such as, value, benefits, members-only events, etc...  With our previous agency, we had 7 different versions allowing alot of flexibility with ads.  

I am concerned that with a lower number of versions, the public could "tune out" our ads.  What does your organization do?  Do you rotate different versions of your ads throughout the year?  How many versions do you have?  Any pros and cons?  Thank you.