Department heads might be at rank Lt. Commander but the other positions would be covered by Lieutenants and Ensigns with officers making up perhaps 15% of the ships crew.

Oh and many forget that there is a rank between that of Ensign and Lieutenant so I thought this chart might be helpful

[hider=Ranks][Img]https://startrekdistanthorizons.yolasite.com/resources/Ranks-ALL.jpg.opt880x757o0%2C0s880x757.jpg[/Img][/hider]