You must request that from customer service.
There is no single published number because it changes contently depending on the nature of the call and agent availability. Following the link below will either give a current number for you to call or take your number for a callback.
(Note that phone support is for paying customers only. There is no phone support for Free Customers.)
Here is a TurboTax FAQ for contacting customer support.
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number