Based on the strength of Earth's gravity and the density and strength of rock, in principle you could make a single conical mountain that stretched between New York and Chicago and soared over 45km! That's twice the size of Olympus Mons and definitely dwarfs Everest. However, there are a couple of reasons why we can't actually have that humongous of a mountain on Earth. For one, Earth's crust is made up of continental plates that essentially float in the semi-solid rock of the mantle below. If you add more weight above the surface, they sink lower into the earth's hot interior, and when they sink far enough, they soften and basically melt. For our conical mountain, that gives a new height limit of just 15km. As well, the powerful collision of two tectonic plates, which creates mountains in the first place, also fractures and cracks the rock, weakening its structure and exposing it to erosion. Over millions of years, freeze-thaw cycles pry at these cracks, while winds claw at the slopes and streams and glaciers carve deep valleys into the mountainside, all weakening the mountain's support. This can end badly. For example, 3,764m tall Aoraki Mount Cook in New Zealand had its top fall off one night in 1991, trimming it down to a 3,754m mountain.