The government is taking a census of its 2.6 million workers to find out how many have physical or mental disabilities.

It's part of a program to make Uncle Sam practice what he has been preaching for years: hire and promote the handicapped. Despite half a dozen government operations designed to monitor handacapped programs in the private sector, federal officials say they have no accurate data on how many handicapped workers are employed by the federal government.

For the past few weeks, thousands of federal workers have been struggling with self-identification forms that request emploees to indicate if they have one of more of 93 medical disabilities listed. They range from deafness to paralysis, from controlled diabetes to scars from burns and gunshot wounds.

Employees are asked to code their own physical or mental problems (if any) on the form, and to supply name, birth date and Social Security number. However, it is possible for workers - who must hand the forms back to hosses - to leave their names and/or Social Security numbers off the questionnaires, or to leave them blank. Nevertheless, some employees have complained that the self-identification exercise is a paperwork-producing project that invades their privacy.

Federal officials say the government is required (under a 1973 law) to find out how many handicapped workers it has, where they work and what they are doing. Old methods didn't give a very good picture of the federal employment picture for the handicapped, they say, but this new form will - if everybody fills it out properly.

According to government estimates, there are only 74.400 handicapped workers with federa agencies now. But officials in some agencies say estimates have jumped 10 per cent already on the basis of partial returns.

Once the information is in, the Civil Service Commission, apparently using Social Security numbers, will be able to see which agencies are doing the best (or worst) jobs in hiring, placing and promoting the handicapped.