Jersey numbers

Depends. What is your purpose for doing so?

If you do this on your shirt:

http://images.collectors.com/smrweb/smr0205/mantle.jpg

And wear it, your are probably fine.

If you do this:

https://images.footballfanatics.com...roductimages/_114000/ff_114450_full.jpg&w=340

On lots of shirts and sell them, you will probably get sued.

 

Numbers are not trademarked, but team and league logos are.

 

If you authorize someone to create a shirt for your personal use alone, I doubt anyone would come after you.
If someone were to be pursued, it would be the person who created the shirt.

However, how would anyone even know you're wearing such a shirt, or who you are to even approach you about it?

If you don't answer, no one would made know who made the damn shirt.

Even if you are asked about the shirt, you're under no legal obligation to respond.
In fact, I never answer questions.

Blabbing is how most people get into legal jams, their dangerous tongues.