why is it said that total charge of an isolated s

Charge is conserved for an isolated system means that the net charge of isolated system remains constant. Only way to alter it is to introduce or remove charge from it.

Take an isolated system of two seperated identical spherical ball A and B with charge +4Q and -10Q respectively. Now bring them close and allow them to touch. What will be the charges on the ball? Both balls will have now -3Q charge. (i.e. net -6Q ). No matter what operation you would do in this isloated system of two balls net charge will always remain -6Q.

To understand isloated system while problem solving just draw an imaginary boundry around the charges and that becomes an isloated system. Nothing can come or go outside of it.