My wife and I are up to 8 prime cards and she holds one store card as well. My wife will admit that the current card load is making her nervous in terms of making sure everything is organized. It was a lot easier for her two months ago when we just had three cards to manage. But after March, one of the cards will likely get sock-drawered (except for occasional purchases) since better cash back deals are on other cards. We'll see how it all goes.
I do agree that a card app needs to be for a product that you are going to use past the introductory bonus. I was all for getting a CSP until I realized that I would never use it past getting the bonus. I don't need another travel card...I travel for a living anyway, and all those purchases are on a company provided Amex not listed below. All the new cards have a specific purpose for saving money in our budget this year. I'm looking for a $2500/yr spiff over time based upon our previous purchase history.