As of December 31, 2017, President Trump had filled fewer government science positions than either of his two predecessors, at the same point in their respective administrations. Of the 83 government positions designated by the National Academies of Science as “science appointees,” just 20 had been filled by President Trump as of December 31, 2017. At the same point in their respective administrations, President Obama had filled 63 such positions, and President George W. Bush had filled 51.
Update:
As of August 2018, President Trump had filled 25 of the 83 government positions designated as “science appointees.”