according to PMS rules the total number of posts in grade 17 is 478.
including

Section Officer 200
Assistant Political Agents 25
Deputy District Officer (Revenue) 50
Deputy District Officer (Judicial) 30
Human Resource Development Officer 24
Secretary District Public Safety Commission 24
Deputy District Officer (Finance) 48
Tehsil Municipal Officer 54
Secretary District Regional Transport Authority 24

Secondly to be promoted to Grade 18 one should have at least 5 years of service in grade 17, which means non of the previous batches are going to 18 in ongoing year.

Thirdly there are a lot of serving non PMS officers holding these posts.