How many flexible tracks does it take to make a complete oval?
Each piece of flex track is equivalent to 1/4 piece of regular track. The standard train curves are 1/16th of a circle and thus an oval shape is not possible, however, with 64 pieces of flex track you could make an oval by squashing the circle a bit. 7499 comes with 16 flexible tracks, so you'll need 4 of them to make a regular train circle.

The flex track can be used to make a smaller circle than the regular track:



but as TheBrickBlogger pointed out, the flex track has performance issues and such a tight circle will be difficult for your trains to navigate. The trains tend to fall off if conditions are not perfect. Over Christmas I built a large layout under my Christmas tree and I had to remove all the flex track in order to prevent derailing. Part of that was due to my train design, part of it was due to the fact that the track was sitting on a mix of carpet and hardwood flooring, but part of it was that the train is very sensitive to bumps in the track and flex track exacerbated those issues. Depending on your engine design you might have more or fewer issues. If you are just pushing the train by hand, it will probably be fine.

Also keep in mind that the standard lego train car with fixed (not swiveling) wheels can only navigate a certain minimum radius. Lego puts the wheels 10 studs apart so that they can just barely make it around the fixed curves. If you build the small flex circle, or squash a 64-flex-track circle into an oval, your tightest curves will for sure be too tight for those cars to navigate.

Normally Lego sells curved track in an expansion kit. It seems that right now (May 2015) the only kit is the 7895 Switching track kit. That kit comes with 4 curved rails and so you'd need to buy 4 of them to make a circle. If that's not your ideal choice of parts, then Bricklink would be best. Or, use this as an excuse to buy a whole train kit, which comes with track, train cars, motors, etc. You can never have too much Lego.