100V/(2^24) = 5.96uV per step.
ADCs don't work on 100V signals directly tho, so you have to divide it down to not exceed the highest voltage your device can measure. 3.3V or 5V or 10V or 12V, etc.
The lowest priced 24 bit ADC I could find (that a datasheet would open for) was this one
It only takes analog voltage in up to 3.6V, so that 100V input would need to be divided down by about 33x.
3.6/(2^24) = 0.0000002V/step, 0.2uV.
These parts can accept up to 10V, but cost 10 times as much!
https://www.digikey.com/en/products/filter/data-acquisition-analog-to-digital-converters-adc/700?s=N4IgjCBcpgrAzFUBjKAzAhgGwM4FMAaEAeygG1xYwBOAJmpAF0iAHAFyhAGU2AnASwB2AcxABfIvAAsANiQhUkTLkIlyIWlLoAObU1YdI3PkNESQAWlrzFfAK6rSkCrBBEGjMeevOQGACaozCBy0CD8-pwWYAAMEAacbiBsAJ4seJwYOKheQA