You need to add up all the variants of a name's spelling (e.g. Ben, Benjamin, Benedict if you want to include all Bens) and divide that by the total number of baby boys born last year. That gives you the percentage of boys named Ben in a given year.
You can't predict at all. My Mum changed her mind about my mind because of its popularity: I never during my whole school career met another child with the same name. But plenty of children (and teachers) with the name she gave me instead .
We gave ds a name that is foreign, extremely unusual in this country, not even terribly common in my country of origin. There's a boy in the next street with the same name!
South London. The DCs are now at a very multicultural school, so that's not surprising, but I don't know any from NCT etc either, maybe the yummy mummies of Clapham moved on from Jack years ago darling.
In my DS's reception year (abou 50 kids), there is one Jack and one Grace. I think they were the most popular names when current reception children were born. There is one Olivia as well, which I think is the most popular now. Bizarely, one of the duplicated names hasn't been in the top 100 for decades!
My DD has a name which apparently about 300 children were called during the year she was born. Three of them (including her) are in her class of 33 at school.
On the other hand there isn't anyone in the class called by any of the top ten girls names at the time.
My fifteen year old DS has a name that was in the top ten when he was born and has only just dipped below top ten now. (I didn't think or care about such things or know any other children when he was born so had no idea it was popular.) However he is unique in his class and in fact in y10 (150 odd boys), although we do come across plenty of other boys with the same name at rugby, swimming etc.
Neither of them are remotely bothered by the popularity of their names. I often think I am glad DH reigned me in on my first choices - Orlando, Xanthe etc - as actually they might be hard to carry off in the area of Yorkshire where we live.