I came to USA 4 years ago and already accumulated probably 15-20 cards. Few of these are store-credit-cards (Kohls and Target) but others are just regular ones.

Reason is simple - more credit cards - better credit score in the long run (it ruins your credit score in the very beginning tho)

I am using 3-4 daily depending on current rewards and perks and never carry balance (and as a result, never pay interest.) Like I put electronics on my AMEX simply because these purchases are auto-protected from accidents for 90 days. I put gas purchases on card having 5% cash back on gas stations, etc.

As additional bonus - I put not used cards into glovebox of each of my car and in case I forget/loose my wallet, I still have mean of payment with me.