We have an above ground 27′ pool…approximately 20,000 gallons. After shocking the pool on Friday and adding 2 chlorine tabs, today, Tuesday the ph and alkalinity are fine, but the chlorine has dropped to .5 ppm. Should I shock again or add more tab?

Since you now have a residual level of 0.5, the tabs would probably be sufficient. If it drops to 0, it is time to shock again.