My two cents

Often you'll hear qualitative researchers say that interviews or research with 5 people from the same target group is already enough to have a good understanding of that group.

But then you could ask yourself:

Isn’t there a larger number you should use to validate the problem compared to 5 when validating the solution?

Here's how I've worked with that challenge in the past:

The way I think of it is as follows:

Basically, I try to find interesting problems to work on, and then verify that it's a problem for enough people so that it's worth working on it.

For that second part getting help from someone with knowledge of quantitative research and statistics is pretty helpful. I've used these three approaches:

For surveys, I use the Sample size calculator created by the team of SurveyMonkey.

This tool helps to know to how many people you should ask the question (sample size as the data nerds would call it).

As I said, I suck at quantitative research. But here is one thing I learned: even when you feel the data tells you something pretty clear, you have to verify you didn't get these results only by chance but that if we repeated the research, there would be a good chance that the same result would come out (what data nerds call Statistical significance).

To verify that, as I'm not yet to the level of someone who knows statistics, I just get a freelancer to help with the analysis of the statistical data.

For landing page testing, where you want to know if the interest in the product, or service is big enough, I try to compare the percentage of people that sign-up with similar products or services (conversion benchmark, as the marketing nerds would call it).

I've collected a list of conversion benchmarks in my notion bookmarks here.