keep 3 to 5 credit card.
apply cc is like a "ping" to your credit. It will reflect on your credit history but will not do too much of damage in terms of score. Only if the frequency is too offen that will low the number.
Laws in states like Colorado, Georgia, Massachusetts, Maryland, New Jersey, Vermont require credit agencies (big 3) to provide you with a free copy each year upon request.
I think the length is somewhat important. But the status are much more important. i.e. pay it before deathline.