"PPM" is a very poor unit of measure, desipite its wide use. Most of the time this is a mass/volume measure, but it's never clear. On the test kits, I think it measures mg/L.
If you want 5 ppm NH3 in your tank, let's do a simple calculation.
The density of pure liquid NH3 is 0.682 g/cc, or 682 mg/cc.
10 gallons * 3.7854 L / gallon = ~38 L tank.
38 L * 5 mg NH3/L= 190 mg NH3 in your tank.
Now divide by the density
190 mg / 682 (mg / cc) = 0.2786 cc of pure NH3 in your tank to get to 5 ppm.
Drop size varies, so take some AMMONIA and see how many drops it takes to fill up a teaspoon. Let's say it's 20. Then you have 20 (drops/teaspoon) / 4.928 (mL/teaspoon) = ~ 4 drops/mL.
Now you have 0.2786 mL * 4 drops /mL ~ 1 drop!
This is for pure ammonia. You will have to recalculate if you're using comercially diluted ammonia. Find out the concentration of your ammonia solution. If it's 50%, multiply the number of drops by 2 for a decent approximation.