Ronald Reagan will be able to fill about 6,000 top government jobs -- at least half of them at the $50,000 pay level -- now that he has taken over as manager of the free world's largest business, the U.S. Government.

In addition to the large batch of political and policymaking federal slots that normally turn over with administrations, Reagan will be the first president with the power to make substantial changes in the makeup of the Senior Executive Service, which currently has 7,062 members. The 120-day job security countdown for the 6,419 career members of the SES began yesterday, as soon as Reagan took the oath of office.

The SES was created by Congress and the Carter administration as part of civil service reform. It set up an elite cadre of officials from the old supergrade (Grades 16, 17 and 18) ranks who are supposed to get bonuses and better pay. In return, they give up much of their job security, and are much more vulnerable to transfers, or dismissals, than federal types outside the SES. SES is authorized about 8,500 jobs. That means the Reagan team will have a major say-so in who gets hired for about 1,200 vacant $50,000-a-year jobs.

SES now has 6,419 career executives, 569 noncareer, 58 with "limited term" appointments and 16 in emergency-type appointments. All but the career people can be replaced immediately by the new administration. In addition, the Reagan team has about 2,700 so-called Schedule C jobs it can fill. The Schedule C system is used to identify individuals, from senior cabinet assistants to cooks and chaufeurs to the mighty, who work at the pleasure of their immediate political boss.

Except for cause -- that is, criminal or related misconduct -- career SES members cannot be replaced by their bosses unless they get bad performance evaluations. Under terms of the CS Reform Act career SES personnel, most of whom live here in Washington, cannot be given a new performance evaluation until 120 days after a new president takes office. That means the SES types and their new political bosses have a 120-day getting-to-know-you period (it expires May 19) before the new evaluations can be made. Nobody is anticipating that the Reagan crew will decide to kick out large numbers of SES officials at the end of that time. But the CS Reform Act makes it easier!

There is a second 120 countdown that will vary from individual to individual and from agency to agency. It involves the ability to transfer people involuntarily. (One way to isolate or force a top bureaucrat to quit is to stick him or her in a dead-end job, or shift them to Kodiak, Alaska, or somewhere.) That sort of involutary transfer cannot take place until 120 days after the political boss of the career SES member takes office. So there are two countdown periods to watch; the 120 days that started yesterday for all career SES members and the second one, covering involuntary transfers, that will begin ticking immediately for some, later for the others depending on how quickly they get a new political boss.

Demotion from the SES doesn't mean poverty or unemployment. Unless the action is taken for cause, the SES member returns to a career Grade 15 slot which, for the moment, pays the same $50,112.50 that most SES members get. But such a demotion would not be without emotional and career scars. A demoted SES member, for example, is put on notice that he is stopped on the career promotion ladder. And it also opens up his/her key job for someone more cooperative with the team running that agency or department.

With their last breaths, Carter officials maintained the new system created with the SES need not lead to politicization of the upper reaches of the career civil service. True. It doesn't have to happen. On the other hand, the changes Carter and Congress made will make it much easier for the new administration to bounce people out, transfer them or otherwise shift gears and personalities at the top, if they want to do it.