Note: Excel has several built-in data validation rules for numbers. This page explains how to create a your own validation rule based on a custom formula.
To allow only numbers in a cell, you can use data validation with a custom formula based on the ISNUMBER function.
The ISNUMBER function returns TRUE when a value is numeric and FALSE if not. As a result, all numeric input will pass validation.
Be aware that numeric input includes dates and times, whole numbers, and decimal values.