Question

How da value of constant in Newton's second law came?

Answer 1

Physicists love making system of units where constants which appear frequently is equal to unity. Newton's second law is one of the first examples of such a case.

Now you must have seen this version of the second law
F⃗ =k⋅ma⃗
In S.I. units the unit of force is Newton. So you define 1Newton as the force that is required to accelerate a mass of 1kg by 1m/s2
When you use this definition k=1.

On a sidenote there is a system of units favoured by high energy physicists called where the three fundamental constants of nature ℏ,c,G are taken to be one.