I know that battery has a pd even when it is not in use... this means that there are more electrons in negative terminal of battery but how more?? Does it depend upon the battery or electrodes used or any other factor?
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The number of electrons will be very small and is usually ignored. For an estimate consider a glass cylinder (dielectric constant $\approx 3$) the size of a 1.5V AA cell, with end area approx $2cm^2$ and length $5cm$. The capacitance formula $C=\varepsilon \frac{A}{l}$, with $\varepsilon=3\times9\times10^{-12}$, $A=2\times10^{-4}$, and $l=5\times10^{-2}$ gives $1.04\times10^{-13}F$. The charge will be given by $Q=CV$, and rounding the capacitance down gives us $1.5\times10^{-13}$ coulomb. A coulomb is about $6.2\times10^{18}$ elementary charges, so ths gives us $9.3\times10^{5}$, or about a million electrons.
This is about $\frac{1}{6}$ the charge of $1$ microamp flowing for $1$ microsecond.
The above analysis is rough. For a start it depends on the particular battery, and the geometry of the battery components is not what I assumed. But it should be good enough for a rough estimate.
