What is the maximum number of tags a mob can have?
Solution 1:
No, you don't need to be worried. Look carefully:
Target either already has the tag or has too many tags.
Here is what happens:
First run
Minecraft adds the tag test to the player.
Second run
Minecraft tries to add the tag test to the player but it already exists. So it throws the error.
I don't know the limit off-hand, but it should be more than you ever need.