Suppose I have a spring balance kept horizontally on a table, and there is a block of mass hanging from it say from the edge of a table through a frictionless pulley. Let’s say the spring in the balance too has a non negligible mass. Does it affect in calculating the weight of the body? Like if it says “N newtons” in the scale, is it actually N newtons? If not, how?
If you neglect the friction between the spring and the table (which depends in part on the weight of the object), and the system is in equilibrium (you give it enough time for it to stabilize), then the mass of the spring is not important.
A spring balance is meant to weigh loads substantially exceeding the mass of its spring. If this condition is met, the difference in the readings between vertical and horizontal orientations of a spring balance should be negligible.
With this in mind, if the pulley in your setup was frictionless, the spring balance would show the actual weight of the mass (within its accuracy). A real pulley has some friction, so, in a real setup, the weight indicated by the spring balance would be a little lower than actual.
If a spring balance had "a non negligible mass", its readings would depend on its orientation. If such spring balance was calibrated for the vertical orientation, a correction would have to be applied when it was used in the horizontal orientation.