I dont know if anybody else found the information I was looking for but here it is.
If you want to know how many empty mech bays you still have go to the store on the upper right menue on the main page. Then select mech bays.
If there is no number shown as empty mech bays - you dont have any mech bay left/free/unused.
I just bought some mech bays because they are on 50 percent sale atm (till 23rd June I think) and look, when I bought my first mech bay - there was shown a little textwindow within the little mech bay purchase "Icon" which says "Empty mech bays: 1" (2, 3, 4 etc. depending on how many mech bays you have empty

I hope this helps a lot to everybody
greetings to everybody
Yamabushi Giri from the Shieldwall of Rasalhague aka The Skjaldborg