Tip speed governs max Nr, as already stated.
The aircraft I trained on (Whirlwind 10, three long, spindly blades and a single Gnome turbine) had a normal Nr of 210 rpm (I think, it was three decades ago) and just 190 rpm / 80 kts (more certain of that) for a range auto. At 190 rpm you could see the individual blades passing.