Sorry, I did not proofread well. I said:
I tried to say +M5, but that resulted in an error.

I *should* have said:
=M5. That is, I tried to set  the maximum  value to whatever value  Cell M5
has.

I do not think you can set  it to a cell  value. You will probably have to use
code to set the value  - perhaps when you initialise or show the form.

Not sure of this item is in a form. I tested this macro with a 'spinner' form
item placed on a worksheet:

Sub SetMaxValue()
vNameBoxValueForShape = "SpinnerX2"
vLimitValueInCellM5 = Range("M5").Value
ActiveSheet.Shapes(vNameBoxValueForShape).Select
Selection.Max = vLimitValueInCellM5
End Sub

To automatically get the value  from M5 into the Shapes control  MAX value, you
will need a worksheet 'Change' event that fires when the value in M5 changes.