Numbskull is an English word I don’t hear so often these days. I am using Stack Exchange for an explanation of its meaning as my usual source, Merriam-Webster, has a rather uninteresting entry.

The forum exchange included these comments:

If the OP wishes to express someone’s ignorance of math, I would suggest using a different expression.

I’m a complete idiot when it comes to math.
Alternatively, the noun numbskull is also very appropriate.

a stupid person; dolt; blockhead

I’m a numbskull when it comes to math.
Source: The Free Dictionary

Then there was this contribution from a Brit:

I can’t think of a standard animal/object/phrase that would naturally fit the gap in I’m a ___ when it comes to math except for something dull like I’m a dunce when it comes to math.

If you’re looking for something a bit more poetic, in British English, you can combine the informal colloquialism thick (meaning stupid) with a standard simile for emphasis and comic impact, e.g. as thick as two short planks or as thick as mince (the latter is perhaps specific to Scotland).

So if you wanted a nice idiomatic, albeit very informal, British English way to express your innumeracy you could say:

I’m as thick as two short planks when it comes to maths.

(Of course in British English we say maths rather than math.)

So, as you can gather a numbskull refers to a brainless person. As the last contributor says, in Britain we would say about such a person, “they are as thick as two short planks.”

Descriptive, don’t you think? 🙂