Here's the backstory.
Over the years we’ve tested many sample sizes - from very few to hundreds - and found that having a panel size of 50 consistently yielded good, high-quality feedback. Having a group of 50 people is the best balance for price, quick turnaround on answers and a non-skewed audience, especially for iterative testing. Our users run an average of 3-5 polls per creative asset (packaging, product design, listing photos, book cover art, etc) when performing their market research.
If you’ve run your polls with 50 respondents and decide you want more, you can always add additional respondents to your poll by completing the steps that can be found on this help doc.