ATR: max altitude is FL250. Cruising high saves fuel. For the ATR72-200/212, for each 2000ft above FL180, fuel burn decreases by between 16 and 24kg/eng/hr depending on a/c weight and cruise FL ISA deviation. This is valid both at 77% NP (non-icing conditions) and 86% NP (icing conditions). In a nutshell: CRZ fuel burn will be around 600 kg/hour or less above FL210.
A graph in FCOM 3.04.02 shows the max FL that can be reached depending on weight and outside temperature (expressed in terms of deviation from ISA). This is worked out as the max FL the aircraft can reach with a residual rate of climb of 300 ft/min in normal conditions, and 100 ft/min (!) in icing conditions (IAS climb).
That said, choosing a high CRZ FL has to be weighed against sector time and headwind gradient. According to ATR data, should the wind gradient be less than 6 kts per 1000 ft increment, then climbing will save fuel, with very little increase in trip time.
Cheers