Question

Will the momentum remain constant if some external force acts on the
system?

Answer 1

It can differ as the external force is very strong then it can cause a disturbance in the momentum of the object .Eg:- A rolling football can be kicked and the direction and the velocity can change.

Answer 2

No, the momentum changes if External force acts on a system
F=ma
p=mv
F/p=a/v
=> p=(F.v)/a
Momentum will be directly proportional to external force