On a standard tube setup, Slime is a good option to prevent flats. The Slime will work on a puncture of up to 1/8 (3mm) on the thread area. If the puncture is bigger, elongated or outside the thread, Slime won’t be too much of use as it’s not guaranteed to fill gashes or cuts only punctures.
How much sealant will depend on the trademark you are using. Slime recommends around 4 oz of sealant per tire on a 26" x 2" wheel.