Sky's the limit. Theoretically, you could have a very high or low number, but this is limited by the materials they are made of and what you could reasonably put on someone's face. I've seen glasses as strong as +15.00 and -35.00 in my own experience.
.25 to infinity. The smallest amount of correction we measure and put in glasses or contacts is in .25 diopter increments. The highest power that I have seen is -25.0! this person can see about 1/16th of an inch in front of themselves!
+6 to -20. There theoretically is no limit but practical limits are about +6 to -20. Beyond these ranges, contact lenses, implantable lenses or elective cataract extractions are realistic alternatives (refractive lens exchange).