Sims don't change careers to fill gaps, if they already have one.
EA Careers only pulls sims into careers to fulfill coworker/boss positions for your actives if that sim is unemployed.
Ensure that everyone already has a job, and they won't be bothered by the system.
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However if you want to know how many sims are necessary :
Most EA Careers require four coworkers and a boss for each active sim with a career.
Inactive sims are not assigned coworkers or bosses by default. Under EA Standard they do not gain job performance in the same way as actives, so do not require them.
NRaas Industries: Sims 3 Mods for the Discerning Player, hosted by The Wikispaces.