It should be noted that number of credit cards does not determine the credit score. It all depends on how you use your credit cards. 

If you happen to make all your payments through a credit card in order to avail offers and discounts, you should watch out your credit utilisation ratio. Maxing out the credit limit will affect your credit score. In such a scenario, you can get a second credit card to share the credit utilisation ratio and keep your credit score intact. 

If you keep a low credit utilisation ratio even with one credit card, you can definitely maintain a good credit score. At the same time, you can also have multiple credit cards and maintain a good credit score. 

On-time payments, low credit utilisation ratio and paying the bill in full are the important factors that are good for your credit score. 

Additional Reading: How Many Credit Cards Should You Have?