There are two sides to this discussion -
sample size. This is something that Nathan and Syed have covered adequately.
sample selection. This is something that's actually more important.
Suppose you are doing this for a consumer product. Different possible user groups might be college kids, employed people, men with kids, women with kids, retirees, etc., etc. You need to work this out based on the product in question.
These groups vary wildly in their purchasing power, interest level in different types of products/services, etc.
If you talk to 400 college kids only (low purchasing power, enough spare time, highly sociable, interested in fads and novelties), you might get a very very skewed perspective. And if this research were for a home mortgage product, your results would be summarily meaningless.
It is more beneficial to talk to 10 highly targeted/relevant users than to talk to 1000 random people - unless your subject of research is something extremely generic.
For many highly targeted groups, you probably will find it extremely hard to get to 400 users to talk to you. Sometimes these many users might not even exist. E.g. investment bank directors, governors of provinces, etc. If you are talking about a software product that solves a specific problem a group like this has, it might suffice to have meaningful conversations with 10-20 people in this group.