She does only have one (as far as we know).
Actually I don't know if she designed it herself..
It's been a couple of years since she got it. Back then, she was with Tomoya Nagase. They did both get the half of a heart on the back (ayu) and the front (Tomoya) of their shoulders:
(To me, ayu's is beautiful, while Tomoyas fails
)
Then in 2008, in the year of her 10th anniversary, she got it changed to her 10th Anniversary logo:
The original logo looks like this:
Personally, I think the old tattoo was so much prettier, but I also understand why she wanted to change it
(pictures from Google)