No, you always get 13 days off (unless prorated lower by military leave or some other PLA)
You get fly days on top of the 13 days off.

So if you get 3 fly days for the month, you actually get 16 days off.

However you are expected to either fly those 3 days off during that month, or make up those 3 fly days somewhere else during the year.

You will be given 30/36 (NB/WB) Fly days every year. You can fly them in the month you are given them, or during another month throughout the year.
Deadhead from DEN-EWR or DEN-SFO in order to be in place counts as a day of flying as well, and you get PS First Class to do so, unless you DH deviate.

It can be hard to play catch up since we can't go less than 10 days off per month.
However, you can use vacation months to catch up as you can do fly days over vacation.