From what I've understood, it doesn't need to be 5 dimensional, but rather, 2 dimensional.
The dimension in a matrix doesn't correspond to the number of rows and columns. In your case, two dimensions seems enough: one for the severity (from one to five), and one for the likelihood (1 to 5).
You could therefore store data like so :
Matrix[2][5] = 3 (which would mean that there are 3 questions that have a severity of 2 and a likelihood of 3).
A good visual representation of the dimensions of a matrix would be the following. A one-dimensional matrix is just an array/list. A two dimensional array corresponds to an Excel/Numbers spreadsheet. And a three dimensional array could be use to store the coordinates of the cubes composing a Rubik's Cube, for example. Above 3, it's hard to have a visual representation.
A 5-dimensional matrix would be suitable if you had 5 different sliders per question.
Therefore, if you can represent your table in a spreadsheet, it means that two dimensions is enough.
Let me know if my answer is suitable for your project! :)