Depends on what Congress decides the federal government should be doing, which in turn depends on who people elect to Congress.
In general, the number should be sufficient to meet legal obligations with a bit of lag as it goes up or down as agencies and programs come and go or find their scope changing. If the departments cannot meet their legal obligations because they have too few people (consider if tax return processing takes a year), then staffing probably needs to increase. If most people are idle or grossly underutilized, and the work doesn't have a high seasonal variance like tax processing, then the department or agency should be reduced in size. If the work is seasonal and requires low expertise, then hiring can be seasonal too.
In general, the number should be sufficient to meet legal obligations with a bit of lag as it goes up or down as agencies and programs come and go or find their scope changing. If the departments cannot meet their legal obligations because they have too few people (consider if tax return processing takes a year), then staffing probably needs to increase. If most people are idle or grossly underutilized, and the work doesn't have a high seasonal variance like tax processing, then the department or agency should be reduced in size. If the work is seasonal and requires low expertise, then hiring can be seasonal too.
It's not that complicated.