hello everyone

I am making a 3D game, and i am working on the physics. I know the direction of the impulse, now i need to know its magnitude. I have looked at conservation of momentum, but i don't know how to apply that if the object can rotate.

so my question is, how can i calculate the magnitude of an impulse at a collision?

I don't think the conservation of momentum is the right approach. For your collision resolver you should have coded in something to represent the Coefficient of Restitution. With the surrounding math regarding closing velocity and everything else, you will be able to define how 'sticky' the collision will be. As an example, let c represent the Coefficient of Restitution, and s be the separating velocity, then:

if c = 0, then s = 0. The objects colliding will stick together at the point of contact.

if c = 1, then s = s. The separating velocity will be the same as the closing velocity.

and how about rotation? if a object only rotates and hits a object, the object will get a impulse either. and what if the object both has a linear velocity and a rotational velocity?

You're getting into the realm of the 'moment of inertia'. You will need to also simulate friction. Do you have any idea how to move forward with that?