First off - don't trust CreditKarma to get a perfect picture of your credit score. Pull a real FICO - you can get a free one a variety of ways. Your real square can easily fluctuate (often for the worse) by ~25+ points from CK.
Credit Karma, for example, puts me at 820. Whereas a real pull put me at 805.
Secondly, you're right - more credit cards just equal, assuming you don't spend more, a lower revolving utilization. The more "available credit" you have, looks better for your score and odds at being a good loanee.
Each new CC does a hard pull on your account. 1 a year is no biggie. 2 is pushing it a bit, especially if you are looking to buy a car or a house. That said, pulls don't terribly effect your score.
The problem with CLOSING a CC is two fold, really: One, it's absolutely going to lower your available credit, and thus also likely demonstrate a higher utilization (even if you pay off regularly). Secondly, if it's an older card - well, now your credit HISTORY isn't as long. And CC history actually does have a pretty significant effect on your score.
Don't go open a new card every six months for kicks. That's a bad idea. But assuming you don't see credit cards as "free money," there's no harm whatsoever in having a bunch of cards.
Like you, I got one right as I turned 18. Another a few years later. Another a few years later. AT 27, almost 28, I have five cards. But just got married, and my wife has four. So that puts us at nine cards. Together, we have about $200,000 of available credit, lol. Thing is, we usually put about $3,000 on one or two cards each month and pay them off, and another ~$3K on business expenses that I get paid back on - so yay points.
Just be mindful that if you have too many cards, and end up not using one for an extended amount of time, the card may be closed on you without your input - which has a negative effect. So even a shit card, I'd use it once or twice a year.
Lastly, if you do have several cards and one is just driving you nuts - you'll never use it - it's likely not going to kill you if you close it. The temporary hit to your credit score is better than either being tempted to overspend or making otherwise bad decisions with credit.