UX testing can be performed on 71 users and give very good correlation.
It depends on the tests, the question/task set etc and the diversity of the pool of users (e.g. you couldn't ask 100 people in a pensioner home to carry out a task and think it applies to the whole population). This is very different from a statistical survey based upon opinion.
UX testing with a pool of just 5 users can provide sufficient result for your own apps to be clear about consensus if you have a clear problem you are trying to address. It may not suffice for a general subjective or opinion based question about your website or app.
Heatmap testing general requires around 40~50 participants.