You can keep track of how many credits you have in your dashboard...
Or, anywhere in SeekOut by clicking on your name in the top-right corner of your SeekOut screen.
For more details - including when your credits expire - click on that Credit portion of the menu. Here, you'll see "credits available through the XX day of the month.
Credits do not roll over from month to month.
If you purchase any add-on credits, however, they will never expire.