I actually don't think you can have too many playmats. If I'd have two playmats with same artwork, I'd sell one of them. I don't own many playmats as I'm quite new to competitive TCG. Here is my playmat "history":
I got my first playmat with Ho Oh when I was the only Senior at a League Cup. Later, I traded it for a Regionals Necrozma playmat and some other cards.
After that, I got two Kartana mats at a Regionals, one of them being a Top 8 mat. I sold the "regular" one for around 50$.
Sadly, I lost all playmats when I came back from a League. (I assume they're not stolen because my $200 deck was in the same bag, and it didn't disappear).
Then, I ordered a custom playmat just to have one. Recently, I got a DM Necrozma playmat and I won a Leafeon playmat, which is my favourite.