On a standard tube setup, Slime is a good option to prevent flats. The Slime will work on a puncture of up to 1/4" in diameter (~6mm) on the thread area. Slime will not help if the puncture is bigger, elongated, or outside the thread, as it’s not guaranteed to fill gashes or cuts - only punctures.
How much sealant will depend on which brand/model of tire you are using? Slime recommends around 4 oz of sealant per tire on a 26" x 2" wheel.
For more information: Slime Tire Sealant