You can only give each individual in Eventree one Pass from each Pass category e.g. 1 Artist + 1 vehicle. This is an intentional design to stop people from being assigned multiple passes of the same type which they can distribute to their friends or sell to other people. One of the advantages of using Eventree is that you know exactly who you're expecting at your event.
That said, there are 2 ways in which you can give multiple passes to people:
Create a new Pass category (for more information on this process please read this article):
Now click on + Add pass
And then create your new Pass type making sure it's added to this category. Click Save afterwards
3) You can now assign this new pass type to individuals who need an additional pass.
2. Put the individual in Eventree twice (workaround)
This is a bit of a workaround - but if you have already distributed lots of passes and you later discover that someone in Eventree will need two passes from within the same category, you could add an individual into the system more than once, so if you really want to give someone multiple passes just put them in Eventree multiple times.
This, however would not work if you have 'Unique Individuals' ticked.