The number of ex-federal workers drawing unemployment benefits (39,000 as of the last week in December) could jump dramatically in the next few months as federal agencies step up budget-inspired layoffs.

Ironically, the longer agencies wait to fire people, the more people they have to fire to accomplish the same savings during this fiscal year.

Agencies that could get away with limited reductions in force (RIFs) early in the fiscal year that started Oct. 1 could be forced to fire seven times as many people if they wait until early summer.

Because federal agencies must pay severance of up to one year of salary, lump sum leave benefits and unemployment benefits of workers they fire, the first-year costs of a RIF can be substantial, particulary if a lot of the fired workers are people with high salaries, long service and big annual leave accounts.

Some of the upcoming layoffs planned by federal agencies will hit upper-grade professionals hard. That is because programs are being cut and many of those programs are administered by high-grade workers.

Furloughs: Mindful of the heavy first-year costs involved in RIFs, many agencies may decide to make cutbacks in S&E (salaries and expenses) accounts by furloughing large numbers of workers, rather than firing a few. The Office of Personnel Management, the agency in charge of monitoring furlough and RIF actions of other agencies, is planning such actions for substantial portions of its staff.

Many departments and agencies whose final budgets for this fiscal year have still not cleared Congress are under orders to trim fiscal year 1982 spending by 16 percent. A few "lucky" agencies, and portions of some departments will only have to cut back 12 percent.

Cutting expenses in labor-intensive agencies (places where most of the funds go to pay people) means eliminating jobs. There will be lots of RIF notices and furlough announcements in the weeks ahead.

Census Bureau on Friday advised its nearly 6,000 workers (most of them here) that there will be a RIF within the next six weeks that will eliminate about 500 jobs . . . General Service Administration has handed out 810 RIF notices to its 11,500 staffers in the Washington region and headquarters office. GSA expects that 325 will be fired, and the rest will be moved to other jobs, demoted or otherwise affected by the bumping effects of the RIFs.