The control number is not required. If you are entering your W-2 manually, and there is no control number on the W-2 form that you got from your employer, just leave the control number blank in TurboTax.
If you are trying to import your W-2 and it is requiring you to enter the control number, you would have to talk to your employer. Sometimes an employer tells TurboTax to require the control number for importing, but then doesn't put a control number on the W-2. The employer has to straighten out the error. But it would be faster and easier for you to just enter the W-2 manually instead of trying to get your employer to fix the problem. Importing your W-2 is a nice convenience, but it only takes a minute or two to enter a W-2 manually.
If your W-2 doesn't have one, it's no big deal. You can simply leave it blank. No control number!