By default, the 'Number' field accepts all numbers. But you might want to:
Particularly, if you'd only like to accept 0, 5, 10, 15, .... 1000, do the following:
Now, try using the up-down arrow keys next to the field-preview. You'll see that the numbers in the field-preview will follow the pattern: 0, 5, 10, 15, .... 1000. The number won't fall below zero and won't exceed 1000.
Please note that each of 'Min. Value', 'Max. Value' and 'Step Size' is optional. You may use all of them, none of them, two of them, or just one of them. It's all up to you.