How many monolingual Welsh speakers are there?

As of 2017, very few indeed. there may be a tiny handful of people in remote areas of North and mid-Wales who find speaking English difficult and struggle with it, but they would not number much more than two or three dozen at the very most. However, a fifth of the population, whilst being able to speak English, use Welsh as their first language, and the number of Welsh people who know SOME Welsh, even if it's only a few words and phrases, is somewhere between two-thirds and three-quarters.