It can be a challenge for some people to keep up with more than three credit cards. Some credit card issuers have a policy where they can close your accounts after six or twelve months of inactivity. So, how often should you use your credit cards to keep them "alive"? The answer is that it depends on the credit card issuer.
I'm more conservative when it comes to timelines, so I usually make at least one transaction on each credit card every six months. Rules also vary by state. For example, in the state of California, banks are required to send you a notice before closing the account.
My recommendation is to use your credit card every three months to be on the safe side. Maybe I'm weird, but I think about the calendar year in quarters. The one upside about credit card spend is that there's not a minimum requirement to keep the card active. You can buy a cup of coffee and rotate the cards.
I usually add all of my cards to my Amazon account and buy 50 cent Amazon gift card reloads on them. If I have 30 credit cards (I don't), that comes out to $15 per quarter. Your number might be lower because most likely you'll have fewer credit cards that aren't active, and you might have a few cards you regularly use.
Having Amazon Prime and being on Slickdeals.net helps me find great deals. I typically don't buy things unless it's on sale or it's a necessity.
Even if you don't have Amazon Prime, there are similar solutions that you can use. Something my friend does is reload their bus pass in small increments each week with the minimum amount ($5).
If you're the opposite of me and you don't sit in front of a computer every day, you can get a credit card holder and rotate the cards in your wallet.