Hi friend, I hope you are having a great day.
When answering brainstorming questions, both the number and quality of ideas are important. Your interviewer will evaluate you on both topics (think as he needs to complete a form with both).
A strong and simple way to show quantity and quality is bucketing up your ideas before starting to bring them. For example, when asked about ideas to increase sales for a new sandals retailer (real question from an interview), you break your answer using a sales framework: 1) Pricing strategies, 2) Value proposition and 3) Locations and 4) Promotions. Then, bring 2 ideas for each (e.g., for pricing, "1.a. adjust pricing strategy with best elasticity to increase sales, and 1.b. use different groups of products for different price layers". Great ideas use something already seen in the case.
I hope that answers your question!
Good luck in your journey!