Cable television programmers, who have put everything from 24-hour weather forecasts to talk shows featuring nude guests on the air, are discovering the market has its limits.

With two competing comedy channels announcing yesterday that they will merge, analysts and other cable-industry observers say the market for new cable networks has peaked, or at least reached a plateau, after years of steady growth.

The two struggling comedy operations -- MTV Network's HA! The Comedy Network and Home Box Office's Comedy Channel -- said they will start a combined service early next year called Comedy TV, rather than continue a battle that has led to losses on both sides. Neither channel could get enough systems to carry them to attract advertisers.

The announcement follows a similar merger last month of two proposed cable channels covering the legal profession -- "American Courtroom Network" and "In Court." And on Monday, the all-sports Sports News Network called it quits after 11 months of operation and an investment of $20 million by its parent, Mizlou Communications Co.

"The environment {for a new service} is much tougher than it was just a year ago," said Tom Rogers, president of NBC's cable-TV division, which owns the Consumer News and Business Channel and Sports Channel America cable networks. "We're glad we started when we did because it would be extremely difficult to get going right now."

Rogers and others in the business say the most important limit on new channels is that cable operators simply can't carry any more programs on their wires. About 90 national programming networks are competing for a spot on local cable systems, which have an average of 34 channels.

Operators could invest in new equipment to increase their capacity, but they are constrained from investing in more channels by heavy debt loads and by an unwillingness by bankers to grant new loans while Congress is still considering regulating cable rates, said John Mansell, a Northern Virginia-based cable TV analyst with Paul Kagan Associates, a media research firm.

Ironically, the cloudy outlook for new channels comes as advertising revenue for existing cable networks is soaring, making cable one of the few robust segments of the media. Cable networks will collect an estimated $1.8 billion in advertising revenue this year, up from $1.45 billion in 1989, according to the CableTelevision Advertising Bureau.

Cable channels also have a second source of income -- payments from system operators that carry the channel. For example, ESPN, the most widely carried cable network, charges 38 cents per month for each of its 56 million subscribers, while others receive between 5 cent and 15 cents per viewer.

With their eyes on this pie, at least a dozen new channels have announced plans to enter the market next year. The proposed start-ups range from Celtic Vision -- a "niche" channel aimed at viewers of Irish descent -- to such services as the Chiller Channel, the Cowboy Channel, the Food Channel, the How-To Channel, the Science Fiction Channel and Talk Television, a 24-hour-a-day all-talk-show network.

But these new services are likely to have a tough time getting a seat at the table, said Betsy Frank, senior vice president of the ad agency Saatchi & Saatchi in New York: "Just because there's more ad money out there doesn't mean it's being spread to more networks. It's the biggies that are getting more of {advertisers'} dollars."

Indeed, Rogers said it takes about 30 million subscribers for a basic cable channel to make a profit. That is a level attained by only about 20 channels.