Regarding having multiple credit card accounts, it does NOT help your credit score to have lots of accounts, and if you are applying for a car loan or house loan it will actually hurt you, as it decreases your debt to value ratio (or is that increases... inverse relationships get twisted in my brain!). When you apply for a loan, they look at those credit card accounts as potential balances against you, even if they have a zero balance.
Having multiple credit card accounts does actually help your credit score slightly. The reason it does is that it signifies more creditors have found you worthy of credit. Initially when you open a new credit card, you score may drop slightly because it will bring down the average age of your accounts, but over time as the accounts age, you get a slight boost from having more accounts open.
And as I said before, one of the things that matters the most to your credit score is your credit usage ratio. Multiple accounts can often make it easier to get a better ratio IF you use the accounts only lightly and pay them off each month, because you can get a larger total credit line. (It can sometimes be easier to get 4 cards with a $2500 limit on each rather than a single card with a $10,000 limit.)
But as you said, when it comes to getting a home loan or car loan, then credit score is only one part of the equation. Depending on other items, banks can get squeamish if you have too much open credit. But that can be on multiple cards, or on a single card - they care about the amount of open credit that could potentially be abused, not the number of cards it is on. And, the higher the credit score you have, the less squeamish they will be about it all. I have credit limits on my credit cards almost equal to my annual salary, and have never had a problem getting a car loan and just recently refinanced my mortgage without the bank blinking an eye. If they had had a problem, however, all I would have had to do to make them feel better would be to quickly close some accounts (which wouldn't have been a problem, since I don't carry a balance).
There is no one size fits all answer. Credit is neither good, nor bad - how you use it is the important thing. People need to stop trying to put the fear into others over the fact that they are considering holding multiple credit cards, if they use that credit responsibly.
CreditKarma has a good credit score simulator that you can enter different things like closing a credit card, adding a new credit card, increasing the limit on a card, etc and you can see how it will affect your credit score. It's free, and an interesting tool to play around with if you have any questions about how changes will impact your score.