Honestly, I'd say some concerns are warranted. It might be that 28 is her final number, or not, who knows? She might be more experienced, but that's no big deal. Someone will always be more experience than the other. And to be fair, no reason to be intimidated. Chances are that she's not really ranking guys, and that she's not trying to hold you to some standard. If you've had 27 partners in (my guess) of 8 years of sexual activity, there's some kind of a reason. I do wonder what the reason is in her case, but realistically, causes for that behavior map all over the place.
The real reason to be concerned is to wonder why you two are compatible at all. You appear to approach relationships very differently, and that's going to show up in far more places than just the number of people you've each had a history with. You're right, for now is just fun, and you shouldn't look down on her or anything. But I'd frankly be surprised if you didn't feel kinda weird about the whole thing. It's probably extremely alien to you. I'm right there with you. I totally get the idea that there are people who have a lot of partners, but I've had one for like 15 years. If I were to be in a relationship, now, with someone who had thirty or forty, it'd just be a culture shock to me. Not in a way that they are inferior or anything, just I wouldn't understand them on a basic level.
The good news is, with anything like this, time is the solution. It's weird, the longer weird becomes the way of life for you, the more normal it becomes. It's a standard knee jerk reaction to have a way of life that's not yours be seen as lesser due to the idea that your way of life is awesome. The only real solution I've seen work is substantial interaction with people who are different.
You're normal. This is normal. Have fun :) It'll work itself out. Just don't do anything stupid while your brain is adapting. And that's a general life lesson you can take into a ton of situations, like when you find out your coworker you respect is a devout scientologist :)