Hi @Roxroy_Bollers!
There may be a more elegant solution than what I'm about to suggest, but I'll throw out a couple of ideas and see what you think makes the most sense for your project, or see what other ideas folks have...
So, the first time I read your post, I assumed you were issuing a number to a person, but then on second read I thought maybe you were issuing a unique number to each tree? So you can correct me on that if I've misunderstood 
First thought: if you know the number of the tree you're starting from, say you enter "tree 100", then could you enter a repeat loop, where you define a variable that simply adds one to the previous number each time the form looped, so on the next loop it would be 101, then 102, etc?
Second thought: could you somehow use "pull data"? This would be more appropriate if you wanted to predefine the numbers in some way. But seems a bit more complex if you just want a simple incremental number.
I'm sure there are other ways to do this, so maybe others have alternate ideas. But if either one of these sounds like it might work for you, let me know and we could figure it out further.
Cheers!
Janna