I'm removing z-brick from two walls in my dining room and the plaster is chipping about 3/4 inch deep in some places [not plasterboard]and I'm wondering what to use and how many times to apply it? Can it be filled in one coat or should I use multiple coats?

You can use spackling compound. Apply a coat unitl its flush w/ the plaster. When it dries it will shrink. Sand it smooth to the surface and apply another coat. Sand that after it dries again. It should take about 2 coats but some bad areas may need 3.