At a minimum:

1 Amex
1 Citi Visa
1 Chase Visa
1 Barclays Mastercard
1 Visa debit card tied to a bank account with small amount of cash in it (ATM use only)

Sometimes have a 2nd Amex or 2nd Chase Visa depending on hotel needs. I also usually start a trip with 100 dollars or euros in cash as an emergency backup.

I like having a wide range of both banks and networks. I've been places where one bank is rejecting everything but other ones work fine. It often feels random and I suspect is often due to the infrastructure and banks within the country itself, so I like having multiple options.