Having a lot of credit cards can cost you a lot of money

How many credit cards do you have? How many do you need?

One California man needed 1,098 of them to get his name in the most recent edition of the Guinness Book of World Records. But if you want to catch him, you'd better hurry - at last report, he was still building his collection.

If so, his hobby is costing him plenty, because those MasterCards and Visa cards banks used to give away now mean $10 to $20 annual fees at most banks. Thus, many people are trying to figure out how they can get along with fewer credit cards.

Or they may even go so far as to wonder, ''Do I need any credit cards at all?''

Most experts seem to agree: In today's credit-oriented society , whether you like it or not the answer is a qualified ''yes.'' The qualification comes, they say, because the best use for bank cards is not for making purchases.

''We're evolving into a situation where having a credit card is important,'' says Mel Stiller of the Consumer Credit Counseling Service in Boston. ''It's particularly good as a means of establishing a credit rating - as long as you can handle it.'' While you may not need the card for frequent purchases, using it wisely and paying your bills on time can go a long way toward establishing and keeping a good credit rating, something that will come in handy when it's time for a car loan, mortgage, or a personal loan.

''It's always good to have a credit card for check cashing,'' adds H. Spencer Nilson, publisher of the Nilson Report, a newsletter on the credit card industry.

Many people, however, have a credit card for a much simpler reason: Until a few years ago, a bank - or several banks - would send them one without asking. Or the bank sent an unsolicited form that needed nothing more than a signature to bring in a shiny new credit card.

One result of this deluge of plastic was an enormous increase in consumer debt. At the end of October 1981, the American Collectors Association reports, consumer installment credit reached $325 billion.

Another result is that many people have found their wallets bulging with bank credit cards. They don't just have one Visa and MasterCard, they have dozens of them. Now, they have to pay up to $20 a year for each one.

''I've heard of people coming in with several dozen cards,'' said Len O'Connor, a vice-president for consumer credit at the First National Bank of Boston.

If you've been making your own collection of credit cards, maybe it's time to take all those cards out of your wallet or purse, lay them on a table (bring on another table if you have to), and ask yourself, ''Do I really need all these cards? How many cards do I need?''

''Probably one of each major card is enough,'' Mr. O'Connor says. ''You could easily argue that just one is all you need. Most stores today take both cards.''

Mr. Nilson recommends a Visa and a MasterCard. And they should come from two separate banks. ''Bank records can get fouled up,'' he says. ''And you want to be able to present another card in case the store won't accept the first one.'' He also recommends carrying a card from a major department store in your area and one from a national retailer like Sears, Montgomery Ward, or J. C. Penney.

These, plus a major travel card like American Express and a few gasoline credit cards, might leave a person with as many as 10 cards. ''That should be enough,'' Nilson says.

With even this many credit cards, it is still fairly easy to get over your head in debt. Thus, you need to know how much debt is too much. This depends on a number of factors, including income, the amount you're spending on mortgage or rent, and other major expenses such as school or college costs.

In general, however, Mr. Stiller suggests one possible rule of thumb: The money you spend to pay off all debts (including car payments, but not including your mortgage) should not exceed 20 percent of your take-home pay. So if you bring home $1,000 a month and spend $400 on the mortgage, your other debts should not require you to shell out more than $120 a month, which is 20 percent of the remaining $600. If not all of your bills are paid by the month, you can figure this on an annual basis and keep your credit payments to less than $1,440 a year.

If you find yourself paying more than this, it may be time to ponder ways to live on less until you get down to that 20 percent level.

Maybe National Pay Your Bills Week would be a good time to start.