Yes it is bad to have too many credit cards open with outstanding credit lines.
Having too many credit cards open is bad for your credit score and credit history because it makes you look more potentially risky to lenders.
So only open up a few credit cards and use them and pay them off on time.
You don't want to open more credit cards than you can reasonably afford to pay back because if you cannot pay back all the money owed on the credit cards then you get a lower credit score.
A person really only needs maybe 1 to 3 credit cards at most.
Any more than 3 credit cards per person can be too many when it comes to your credit score and credit history.