A number field.
Assuming that you have a range of 1 to 9999 before unlimited and you want unlimited to be sorted as highest then you would represent unlimited with 10000.
At least that’s the way I’d do it.
Or if I was going to make it easy for the client I would have a radio field to select “unlimited” or “set limit” and then a number field to allow setting the limit with a max value. This number field would be conditional based on the radio field. Then I would create an acf/save_post action that would set this number field to 10000 if the radio field was set to unlimited making unseen by the user.
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