I think it's better to keep simplicity. The point of the tooltip is to establish that the tag can be clicked to show that query. To a net-savvy person, it is probably obvious that the tag can be clicked on as a link, but not everyone is that net-savvy. The tooltip helps explain it, in the simplest fashion, that you can click it, and where it goes. The "where it goes" also helps, because there's also a Tag Stats page that could be a possible destination for clicking a tag. It's a practice of good web design there, actually.
Adding the number to the tooltip offers minimal benefit. In general, if you want to know about how many questions have the tag, you're probably curious what those questions are, so you'll just click the tag anyway. However short a time it takes for that tooltip to show up, it's still longer than just clicking the tag and getting the number shown in rather large font. Also, whenever the tag is listed on the right side, the number is right there anyway (when it is greater than 1) so it is excess noise.
There are other options besides the number, but I can't think of one that would be truly useful. The current tooltip works great at identifying that the tag is a link to a specific destination, which is all it really needs to be in my opinion. Anything more would be all cluttery.