Integer input field: special case for unbounded / unlimited

Question

I am developing a form where the user needs to enter the maximum number of allowed things. Zero is a legitimate value meaning that none are allowed.

I need to find a special value to indicate that there is no limit (using a number slider is, unfortunately, not an option).

Which could be the best alternative among the following ones?

• Leaving the field blank means unbounded; note that this may conflict with a user request to make the field mandatory
• Use -1 as the special value
• Use N as the special value
• Use 99,999 or something like that as the special value

Here is a mockup showing each alternative.

^{download bmml source – Wireframes created with Balsamiq Mockups}

Answer 1

You should add a check box that indicates that the number of things are limited.
(Thus: An unchecked box indicates that there are unlimited number of items).

^{download bmml source – Wireframes created with Balsamiq Mockups}

Answer 2

If you change Max number of FOO to Available FOO (or something of that kind) you can use dropdown list with 3 options:

• Not allowed
• Up to... + input field to enter max value
• Unbounded

This is more readable for users unless you have specially prepared audience.