you lose around 1" manifold pressure to every 1000ftabove sea level . a turbo normalized engine will still only produce 29" on the ground at sea level but as you climb the turbo will keep the pressure at 29" by keeping the ambient pressure in the manifold higher than the outside pressure.
a normal turbo engine (turbo arrow etc) will produce higher manifold pressure to increase engine performance . the average GA turbo aircraft will be wastegated to 39" to avoid extra wear on the engine but will be able to maintain engine power to well past 10,000ft barrier until it reaches critical altitude where the turbo cannot keep up with the demand for higher pressure due to the density of the outside air