Most consumers probably have a “go-to” card that they use for everyday purchases, and for some of them that single card might be enough. However, you might benefit from an additional card if you:

Travel frequently. A general-purpose travel miles card or a co-branded hotel or airline card can partially offset the costs of airfare, lodging and other travel expenses.
Own a small business. Business credit cards have targeted rewards programs, business-specific benefits and higher credit limits than consumer cards.
Have the potential to earn cash back in specific categories. One example is a parent who could use a “designated supermarket card” that earns elevated cash back rates on grocery store purchases.
Want a card upgrade. You might find a new card that’s a step up from your current go-to card. In that case, you could keep the old card active and use it for a small, recurring expense (auto-paying for cloud storage or a streaming subscription, for example).
Although Experian estimates that the average American has four cards, the number of credit cards you should have depends entirely on your unique situation.

Having multiple cards is fine as long as it conclusively delivers benefits that you can use (miles, cash back, perks and so on). Just beware of burdening yourself with annual fees and hurting your credit score with excessive applications.