On estimates, how do you determine the number of unit/material to be used on rebars of footing/column/beam by using ASTM A 615 Grade 60 rebars?

For rough estimates, this is usually based on experience and may differ from one building to another;the simple way is to use some thumb rules based on volume of structural members for example pecent of steel (kg/m3): for slabs 0.5-1.5%;

for beams 1.5-2%;for columns 2-3%;ie: assume that a slab has volume of 30.0 m3 and assumed percent is 1% then steel = 0.01*30  = 0.3 m3 ; 0.3*7850 = 2355 kg so you need 2355 kg for this slab (roughly),and you may take a previously calculated and constructed building simillar to what you are calculating now as a benchmark for your estimates.

As per ASTM A 615  table 1 Deformed Bar Designation Numbers, Nominal Weights [Masses], Nominal Dimensions, and Deformation Requirements Bar Designation