The number of customers that you will need to speak to make sure that you have obtained meaningful and actionable insights highly depends on the complexity of your product as well the number of different personas that you are targetting. The rule of thumb according to Planbeyond would be 8 to 10 distinct people per segment.
Do not forget that customer needs are evolving so customer interviews should not be treated as a one-off project but rather as a tool for constant iteration and improvement.