Dear, I don't know what the highest prescription for glasses is. But I can give you some suggestions to improve your eyesight or correct your vision problem. I have a friend who had lasik eye surgery last year. Now she has good eyesight except that she will feel dryness sometimes. You can take off your glasses and look at the distance for few minutes every hour.
I heard that some prescriptions are higher than -30, though they are very rare. But I can't tell you whether it is true or not for I don't suffer this. You can have your eyesight checked by an eye doctor regularly to prevent the big changes and take the doctors advices to improve your eyesight. There are some exercise and eye relaxation that are recommended to improve our vision, you can search on the internet and have a try.
There is no maximum prescription. lenses can be made to compensate for just about any degree of myopia, astigmatism, or hyperopia so that your eye can see 20/20 with them on (provided they are healthy and you are too). do not worry. practically speaking there will always be lenses available that will work for you.