What is the weakest glasses prescription you can get?

No prescription. Plain glass lenses can be worn for those who like how they look in glasses. If you are blurred when you read or look at distances, even a mild prescription may help you. Usually, people can avoid wearing glasses if they have prescriptions of up to 0.5 diopters; some individuals may feel more visually comfortable with even this low of a glasses prescription.
Plain Glass. I really wonder why this question is asked. I suppose you could get glasses with a plain glass lens if you wanted to wear the glasses as a fashion accessory, but did not need any correction for your own vision.