Opinyin is based on a 'cost per feedback' model. The response allowance is used up based on responses to questions as that way you only pay for the response data you get rather than for partially completed surveys, etc. 

On this basis, a 5 question survey sent to 100 people with a 30% response rate would use up 150 responses although it has been shown that 80% of people abandon a survey halfway through so this usage could be lower. 

You can have as many questions in as many surveys as you like but need to consider that the more questions you add, the more question response allowance you potentially use up. 

It's best practice to keep surveys as short as possible.

Let customers provide as much information as they want in as few questions as you can as this increases the amount you can learn from the ever decreasing amount of time you have the customers attention in today's climate. This gets you better response rates and maximizes learning from each engagement. 

Let customers tell you about what's important to them.

Experts in customer experience recommend you let customers tell about the parts of their experience that are important to them rather than be forced to tell you about things you think are essential. 

This does mean that you need to consider that if the customer hasn't told you about something that you think is important and they don't (as they haven't mentioned it), then you are just meeting their expectations on your points as it's been shown that the majority of people will only tell you if you are really good or really bad but not if you just did what you said you would. In fact, it has been suggested it actually puts customers off if you ask them about things they don't think were important.