Anybody here only have one credit card? If so what is your credit score?
While many do, and might have high 700/over 800 scores, it's still not helpful if getting more credit and the best rates is a goal.
Every type of loan (car, mortgage, credit card, etc.,) looks at something in your credit history to determine Y/N/How much. It's (almost) never just the score. Balances, credit used, debt to income ratio, payment histories, that show a lender how you MANAGE credit is often more important, for the big stuff.
If you have a perfect credit history with just one card in use for the last 10 years, all other debts behind you, that says "I can handle one credit card perfectly!" Yay, but not so much. A lender might be cautious, you don't seem familiar with managing debt to them. That caution will show up in either a denial, a smaller credit line, or extra points. You might never know.
If you have the same, keep balances low and on time, but with three cards in rotation, maybe still a mortgage, a lender says, Ok now here's someone who can manage credit. Tossing them a new card or loan? No problem.