This week in my educational journey to be a better user experience analyst, we discussed the basics of user testing, primarly focused on what you should actually be getting out of user testing and the needs around the number of partipants needed to make a it worth your while. As expected, there are conflicting view points in some cases and like many things, the right answer is, "It depends".
Despite that, the most important that came out of it was, its always better to test, even if you don't have the time or budget to have have a giant testing experience with thousands of participants. Obviously the type of application, and the impact of error should be taken into consideration, but even...lets say... 3 participants can help your team make monumental usability discoveries/improvements.
The moral of the story is always test, no matter how many participants you can affort and make testing a part of the development process, not an after thought.