Usually we say that during the start we need high torque to overcome the inertia where as less torque as inertia reduces during its motion. How can one say that inertia changes when it is purely dependent on the mass of system llike I=mk^2
Your question seems like it may be a bit off-topic for this board, but I'll go ahead and answer it anyways.
The reason why transmissions and multiple gears exist is indeed so that cars have more usable torque at low speeds, but the underlying reason behind this is not because there is more inertia to overcome, but rather because the powerband of an internal combustion engine is less optimal at low RPMs. In other words, the engine has low torque at low RPM and high torque at mid-high RPM. If we just had a single gear that was used for all speeds, cars would be incredibly sluggish to accelerate from a stop for two reasons: because of the gearing disadvantage (which is particular property of transmissions), and because the engine doesn't produce enough torque; otherwise it is not inherently more difficult to accelerate from a low speed.
PS. This "inertia" you're referencing from a physics standpoint is actually "momentum". The relevant equation for momentum is
p = mv, where p is momentum, m is mass, and v is velocity. A change in momentum is therefore linearly proportional to change in velocity or mass.