I think it depends on how you created your text box. As far as I know, there is no default text box in acknex... if you use (f.e.) imgui there is already implemented text box with limitation etc.. You can check it out here.

Best regards!

Edit: maybe I misunderstood your question.. to input only 0...99 numbers, I would check for the length on the string (make sure it's numeric), if the length >= 3, then value is 100 and higher.