The number of virtual cards and applications you can create depends on your plan.
If you want to check how many cards you are using, go to Settings.
(You can access your Settings by tapping on your profile in the upper right-hand corner, or the Settings wheel icon at the bottom left of your screen.)
Once in Settings go to General > Account & Billing:
Here you will find information about the plan you are on as well as the number of applications and cards you are using.