I understand that some stones that are more porous than others and one application of sealer/impregnator may not be enough. But how will I know?

IMPREGNATOR
Impregnating sealer; a chemical product made of two parts: a resin (solid) and a carrier (water or mineral solvent) to penetrate the surface of a stone to reduce the natural absorbency rate of the stones it is applied to, providing that the stone’s surface tension allows the product to be absorbed by it

QUARTZITE
A compact granular rock composed of quartz crystals, usually so firmly cemented as to make the mass homogeneous. The stone is generally quarried in stratified layers, the surfaces of which are unusually smooth, and the crushing and tensile strength are extremely high. The color range is wide. Or, a silver-gray, metamorphic sandstone formed in exceedingly hard layers. In some deposits, intrusion of minerals during the formation process created unusual shades of brown and gold

SANDSTONE
Arenaceous sedimentary rock composed mainly of quartz with feldspar as a possible minor component. Sandstones are clastic in origin (opposed to organic or chemical) and the binding agents are mostly calcite, clay and silica. See quartz-based stone