digital multimeter display count range
First, SAR ADCs generally can't convert voltages above the reference, but that's not universally true. For example, dual-slope ADCs often can work with inputs +/- double the reference, so a 100mV reference will allow measurements of +/-199.9mV.
The number of steps is the same as the number of counts, they're two words used (perhaps loosely) to mean the same thing.
If it is a 16-bit ADC and the reference is 2.5000V, then there will be \$2^{16}\$ steps or counts, and each step will be ~38.15uV.
The ADC just divides the reference voltage into 'n' steps (ideally it does so monotonically, exactly evenly - no linearity errors- and without noise), it doesn't care if the step size makes sense to a human.