Hundreds of Washington tax lobbyists, who have spent the past year recuperating from last year's tax revision law, are camping out again at the closed doors of the House Ways and Means Committee. The stakes -- a $12 billion tax increase -- are not as high as they were last year, but the number at the reunion is no smaller.

"People haven't seen each other in a year. Babies have been born, kids have gone to school, people have changed jobs. It was like homecoming," Sheldon Steinbach, a lobbyist for the Council on American Education, said of the House committee's first meeting last week. The public portion of that session lasted about 20 seconds; lobbyists spent the rest of the day standing in the halls of the Longworth House Office Building chatting with each other.

Some of those who will be following the committee's meetings this week and next are hoping to prevent specific tax increases, but a brave few have even bolder objectives: They are trying to restore tax breaks curtailed last year or to obtain new benefits.

Among those undaunted interests: providers of mutual funds, companies engaged in research and development, authors of books, accounting and law firms and poultry producers. Altogether, an estimated 2,500 to 3,000 requests for alterations in the tax code have poured into the Ways and Means Committee.

"They are standing out there falling over one another to get these things repaired or replaced," one committee aide said.

Most of the major proposals would reduce tax revenue, increasing the deficit, and thus would seem unlikely candidates for inclusion in a bill intended to raise taxes. But the tax increase measure and an accompanying package of technical corrections to the 1986 law probably will be the only tax bills considered in Congress this year, and the lobbyists don't want to miss any train.

"Obviously revenue is a problem. Revenue is a problem with everything these days. But . . . I can't believe there will be no corrections of gross injustices in this tax bill, and if there are any, we think ours will be one of them," said David Silver, president of the Investment Company Institute.

The institute is trying to overturn a provision of the 1986 law that requires investors in mutual funds to pay taxes on some of the costs of operating the fund, even though the investors do not actually receive the money. Cost to the Treasury of repeal: $250 million, by one estimate.

For this and other items on the lobbying agenda to be accepted, however, Congress still must write and approve the tax increase bill and President Reagan must sign it into law.

Prospects for both events did not improve yesterday. In a speech, Treasury Secretary James A. Baker III reiterated Reagan's strong opposition to any new taxes. And Republicans on the Ways and Means Committee, maintaining the stance they took in last week's session, declined after a meeting with Baker to commit themselves to participating in writing a tax bill.

Undeterred by such wrangling, interest groups are seeking their relief in someone else's tax increase.

Authors and screenwriters are struggling against a footnote in the 1986 law that requires them to deduct the expenses of writing a book only against income from that book, years after the expenses. The authors, declining compromise proposals by the Treasury Department, hope to repeal the footnote.

Accountants and other professionals are lobbying to pay their taxes on a fiscal-year basis, rather than converting to a calendar year as required by the 1986 law (their taxes would rise during the catchup phase, although the accountants have proposed another way to make up the lost revenue if they win).

A coalition of high-tech, pharmaceutical and chemical companies wants to use the tax increase legislation to resolve a 10-year-old dispute over how to account for research and development costs for companies with foreign operations. The industry, congressional staff and Treasury officials have agreed on a compromise but it would cost $200 million and no offsetting tax increase has been proposed.

A group of poultry companies is lobbying to overturn a tax break that allows such competing chicken producers as Perdue Farm Inc. and Tyson Foods to defer millions of dollars in taxes each year. The poultry pitch has one advantage, from a political standpoint: Adoption of the change would raise revenue.