For most B&S engines today 1 HP for every 20cc
If it's a smaller mower, say a 19" deck, then 4 HP.
If you have a bigger 22" 3 in 1 machine with a lot of torque then you're going to have 5 to 5.5
I had a 3.5hp B&S lawn mower that lasted 15 years. I don't know how many cc, but, comparing the engines of the old 3.5hp and new 158cc I bought, they look about the same bulk.