I've done this in the past. I find the best way is to do it in person. I made a habit of walking on my downtown street between lunch and asking the local business owners about their challenges of their business. I don't know if there's a number...I think idea extraction should become a part of your daily routine. Here are some questions you can ask:
So your business does X?
What is your role in the business?
What does a typical day look like for you?
Can you walk me through the first couple hours of your day? What are the first few things you do each morning?
How many customers are you working with a month right now? What’s been your best month? What’s been your worst?
What are a couple activities you have in your day that you just don’t enjoy?
Thinking about the last couple days at work, what has been the most challenging part?
What do you use excel for in your business?**
What is the most expensive problem in your business?
What’s a problem that you’ve tried to solve in the past but didn’t work for you?
What would you like to do with you mobile phone, but can’t?