This is going to be very simple, I just know it, but I am stuck in a
mind-trap

A working formula which returns a count:
"=COUNTA(2:2)"
This would return something like "27", a count of cells in row 2 with
data, as does"
"=COUNTA($2:$2)"
"=COUNTA(MySheet!$2:MySheet!$2)"

Now I want to have a cell which I enter a "row number" for my formula
to count, i.e 2,3,4, etc

I would like to have something like
"=COUNTA((B5):(B5))"
"=COUNTA($(B5):$(B5))"
"=COUNTA(MySheet!$(B5):MySheet!$(B5))"

..... work, but it does not, and I can see why is does not. I just can't
figure out how to make it work.

Like I said this should be simple, I'm just stuck at the moment