Cards are a great way to show how responsible you are, and can have an effect on your credit score. However, if using one card responsibly is good for your future plans then using two cards won't make any difference in either case because they both will still be based on the same information.
You only need one card to build a good score, but if you want more points on your driver's license and fewer fines then it's time for another picture!
It's a common misconception that you need multiple credit cards to have strong scores. That idea may be rooted in the belief that one element of your FICO score, which is based on how well-diversified and stable (or not) each account appears to creditors over time with different types of accounts mixed together, can only go up as long as all these individual pieces are doing just fine when taken separately - but alas! It actually doesn't work quite like this at all.
If you're looking to improve your credit score, it's important that the cards in question are used for spending rather than collecting. This will help keep utilization low and may lead to an increase of up 10 points when compared with other types of uses on file at dings providers.
Managing your credit wisely is important to improve your score. It's best not to use more than 30% of available funds on any one card, and it helps if you spread out those purchases so that the ratio stays low.
Having several cards with small balances each allows for less risk when opening new accounts or making large transactions in order to maintain good standing with lenders.
The number of credit cards you should have largely depended on your financial situation and personal life. Being able to pay off the balance monthly is difficult when one only has a single card, but owning two or more can become quite overwhelming if handled incorrectly.