A lot has been said in the last few months about diversity in books. I'm all for it - but only if it's realistic.
Let me explain. In the area I live, 99% of people are white. The majority of those are English or Welsh, with a healthy number of Poles as well. Of the non-whites, there are a couple of Vietnamese families, a few Indian families, one or two Chinese families, and one or two black people. If I was to write a story set in my home town with several non-white characters then to people who knew the area, it wouldn't ring true, and it would detract from the story. Similarly, if I was to set my story in London and only had one or two non-white characters, then it would also not feel right.
The key is to be realistic with your demographics. If you do a quick search for demographics you can find all sorts of stats for the place you're writing about, but mostly it's just a case of common sense.
It's a similar case for including LGBTQ characters - if your story is set in a gay nightclub, chances are most of your characters will be LGBTQ, but if it's not then you don't want to include so many that it's unbelievable.
Of course, there are occasions when you may need to go against the demographics, for example, using current affairs, your story may be about Syrian refugees coming to a small English town and trying to settle in. In that story your main characters will be Syrian, and the people they may ask for help may also be non-white.