I'm trying to do things the proper, "lean" way by looking to survey potential users/customers so I can develop a product people want. The survey is out, but how do I know when I have sufficient data? I've heard 1500 at a bare minimum, but my network is very small and I don't know how I'll reach out to that many people! Does it have to be that high?
It depends on how your survey is structured. If you kept it very simple you could have meaningful results with a much smaller response rate. Focus on binary questions (yes/no rather than multiple choice or open-ended), don't use conditional questions (if you answered yes to question 5 also answer questions 6 and 7), and keep the list of questions short.
If you don't want to start over on your survey, perhaps you could ask your "network" to refer other likeminded people to complete your survey. If you could incentivize them to refer you it would be even better.
Is this B2B or B2C?
If you're B2B I'd really recommend talking to these people directly (in person or on the phone). The problem with surveys is you have to be very careful how things are worded or you can get really inaccurate results. It's much better to have a conversation and get some real feedback.
If it's B2C then you might not be able to really talk to enough people to get a good idea but I'd still recommend talking to as many as you can.
Overall though number of responses is probably not quite as important as quality of responses and that can be hard to gauge.