How is the content placed in that cell?
If it's generated by a formula, there's probably a way to revise the formula to always place a numerical result.
f it's entered directly, Numbers will accept whatever you enter, and change formatting to accommodate the type of data entered.
You can use more than one conditional highlighting rule, though, and although it won't prevent entry of a non-numeral, the rules can be used to flag anything outside the acceptable range of numbers, whether it's 'outside' the top or bottom boundary, or 'outside the 'data type' boundary to left or right:
Default 'fill' for the cell is set to 'no fill'.
Numbers accepts the first rule for which the condition is met.
Rule 1 changes the fill to 'White," making no change in the display, provided the colour behind the table is also white.
If Rule 1 has NOT set a colour fill, Rule 2 is called into play, and unless the content of the cell includes an unbroken string of at least seven x characters, turns the cell fill to red.