How does the value of the electrostatic force vary with the value (how many) of the charges?

Electrostatic force is directly proportional to the amount of charge on each body under consideration.

Electrostatic force increases with increase in charge.

Electrostatic force decreases with decrease in charge.

Explanation:

Electrostatic force is the force between 2 charges which can be explained by the Coulomb's Law:

F = K (q₁) (q₂) / r²

Where

F = Electrostatic Force

K = Constant of Proportionality

(q₁) and (q₂) = Amount of charge on each body

r = Distance between two bodies carrying charge

If the value of (q₁) or (q₂) is increased considering the other factors remain constant, the value of F will also increase.

And vice versa.