I don't know if there are any hard figures on how many t-shirt sites are out there, but even all those "funny t-shirts" aren't the *same* funny t-shirts (well, at least most of them aren't

)
In this type industry, it almost doesn't matter how many other people are selling.
I mean, yes, it will increase your competition for that particular keyword phrase, but there are other ways of being successful. If you have UNIQUE t-shirt designs (whether they be funny, animals, artistic, or whatever) that fit a market need and you can market and advertise those designs effectively, you can succeed.
If someone had told me there was 1,000,000 people selling t-shirts online already, it wouldn't have deterred me any. Because no one was selling what
I was selling and nobody could do it the way I would
