When collating a list of FAQs for your business website, the priority should be content relevancy which means that answering the queries of your customers about your business's service and products should be your approach.
Having FAQs that are sourced from local GMB Q&A, other local directories, chat bot queries, search listening tools with questions of local-flavored content and trends, should be your source to create custom FAQs that cover all aspects of your business and the needs of customers. Additionally, for content freshness ideas, you can make use of topic trending tools such as Keyword Planner, Answer the Public, Topicseed, etc. Reiterating, ensuing that you have FAQs that are locally flavored are integral if you wish to be found by potential customer easily.
While there is no specified limitation on the number of words that should be in an FAQ, having an answer that is concise and directly answers the question should be the focus to suit the requirements of Google's BERT algorithm. That said, according to LARK, a short answer should be no more than 256 characters which is anywhere between 50-65 words. In addition, according to another study, to increase the chance of your FAQ being picked up as a voice answer, it should be concise and preferably be limited to 29 words.