The number is 755 embassy staff jobs, and the press has been very misleading about this. Many, if not most, of those positions will be local Russians who hold staff jobs with the embassies. Putin is capping the staff size at 450, which is supposedly the same number of jobs at Russian diplomatic facilities in the US.
Putin is doing this in retaliation to the new Russia sanctions law that Trump has said he will sign, but it is largely a symbolic move by a dictator who has very badly miscalculated how the American system would behave to a new pro-Russia leader in the White House. US presidents do not hold nearly the level of power that he is used to, so they cannot as effectively bend policy to their whims. Moreover, the reaction to Putin's military intelligence operation against the US election system has seriously backfired. Relations between Russia and the US are lower now than they were before Obama left office, and they are likely to stay worse for a long time. The turmoil and outrage over his meddling makes every attempt to deescalate tensions more difficult, if not impossible.
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Putin’s Bet on a Trump Presidency Backfires Spectacularly