How many credit cards is best to carry?

It depends on how well you can manage them. It is advisable not to have too many cards because doing so, it'll be difficult for you to remember the due dates. However, keep 2-3 and manage them efficiently to have a good credit score. One thing you must follow that is, repay the outstanding balance before the due date.

A good credit is important especially when you want to lead a good financial life. Ideally, you should have 2 credit cards, each from a different network with different types of rewards.

Networks

American Express
Mastercard
Discover

Rewards
Reward points
Cash back
Miles

Much depends on the way you handle credit cards. Around 37% of Americans with 750 credit score have an average 7 credit cards. But if you can't handle 7 credit cards, then it's better you stick to 2 credit cards.