More than 20 candidates are campaigning to succeed New Jersey Gov. Brendan T. Byrne this year, and a bevy of nationally known, underemployed media consultants has flocked here for the gubernatorial primary campaign.

Byrne joked during his final budget message this year that the reason for the glut of candidates is that he made his job "look too easy."

The remark evoked laughter from state legislators, at least five of whom are among those campaigning for the June primaries.

Byrne, a lame-duck Democrat, is prohibited from seeking a third four-year term. During his eight years in office, the Democrats have controlled both houses of the legislature. Republicans are hoping Ronald Reagan's landslide in November, including a 370,000,000-vote victory here, will help propel them into state power.

Newark Mayor Kenneth Gibson, who in 1970 became the first black mayor of a northeastern city, was scheduled to add his name to the list of 10 official candidates today, which would make him the first black to run for governor here. Another 10 hopefuls are expected to enter the race by March 1.

New Jersey and Virginia are the only states with gubernatorial elections in 1981. Perhaps as a consequence, the race in the Garden State is attracting prominent media consultants, political experts and pollsters.

After a banner year of directing the strategies of presidential contenders, some consultants seem almost desperate for action. Many have been wooing potential governors for months.

Meanwhile, such celebrated media gurus as David Garth of New York City have been the object of competing clients.

When Paterson Mayor Lawrence (Pat) Kramer announced his candidacy during a six-stop tour of the state on Jan. 28, he had expected to tell reporters that Garth Associates would handle his Republican primary campaign.

But it seems that Attorney General John Degnan, widely recognized as Byrne's choice as a successor, had the inside track with Garth, while at least one of Garth's partners was leaning toward Kramer. Garth was media consultant for Byrne's easy 1973 campaign and for his uphill reelection fight in 1977.

Last year, Garth managed the presidential campaign of independent John B. Anderson.

Since Degnan has raised some controversy by remaining in his state cabinet post, where he is basking in job-related media attention, Garth has made no comment about his intentions. Degnan is considered by most political observers to be trailing Reps. Robert Rowe and James. J. Florio in the Democratic contest.

Florio and Rowe, two of the top three finishers behind Byrne in the 1977 primary, each directed expensive, high-visibility campaigns against little-known opponents in winning reelection to Congress in November.

Rowe has hired Patrick Caddell, the pollster for former president Carter, to work in his gubernatorial primary campaign. Robert Squier of Washington is his media expert. Rowe also has employed Daniel Horgan, a former personnel consultant in the White House and director of Carter's Pennsylvania primary campaign.

Florio's political consultant is John White Communications of New York.

Paterson Mayor Kramer shares the spotlight with the 1977 GOP primary runner-up, Thomas Kean, as the top contenders for the GOP nomination.

He has hired Roger Stone, President Reagan's former campaign coordinator for the northeast, as a political consultant. Bailey & Deardourff, former president Ford's media consultant firm, also is working for Kean, who is given the best chance of winning support from the close-knit group of Reagan organizers here.

Joseph (Bo) Sullivan, who is believed to be worth more than Kean, is threatening to spend as much as $2.5 million in an attempt to escape anonymity and capture the GOP nomination. Sullivan is president of Bomont Industries in Totowa, N.J. The firm produces coded electric typewriter ribbons.

Sullivan has splashed into statewide headlines with a series of lavish campaign parties, beginning with an $18,000 bash at the GOP convention in Detroit last July. He has hired the Chesapeake Media-Management Group of Washington to help sell his candidacy, but whether that and his parties will convince voters he is the best prospect for governor remains to be seen.

Sullivan's media group includes Alex Ray, formerly a political director for the Republican National Committee, and Karl Ottosen, a campaign coordinator for first-term Sen. Alfonse D'Amato (R-N.Y.).

Sullivan appeared on the first television spots of the gubernatorial campaign, which were aired during President Reagan's inaugural ceremonies.

Kean says he is ready to match anybody's spending if necessary but that he would prefer to work within the state's first public campaign financing restrictions for the primary.

Under public financing, candidates are limited to spending $1.05 million on the primary, with the state providing 2-for-1 matching dollars after the candidate qualifies by raising $50,000. Individual donations are limited to $800.

But Byrne has thrown many candidates off-balance by suggesting proposals to "reform" the bill, which was recently passed by the legislature.

He has threatened to veto it if the legislature fails to enact three additional reforms. They would require a runoff for the two candidates with highest vote totals in each primary, an open primary system and a threshold of $150,000 to qualify for public matching dollars.