There are two ways to get more personnel: 

The advantage of buying personnel with coins is, that it is instant. If you start a hiring phase, you will get a new staff member every night, as long as that hiring phase is active.

Therefore: 

1-day hiring phase: 1 new staff member

2-day hiring phase: 2 new staff members

3-day hiring phase: 3 new staff members

Automatically: any number of staff members up to the maximum of the station