This information is expected to come from Form 3922: https://www.irs.gov/pub/irs-pdf/f3922.pdf
You can see that the "Column C" reference is referring to the TurboTax worksheet itself, it's not referring to any "Column C" you might find on some other form or piece of paper. The actual number(s) that go into "Column C" are taken from Form 3922.
If you use the ESPP step by step interview to enter your sale of employer stock - a sale that obviously encompasses several purchases - you're only asked to enter "Form 3922" information. The only time TurboTax should "speak" to you about a "Column C" is when TurboTax does its review of your income tax return and finds that information is missing.
You've told TurboTax that your sale of stock comes from multiple purchases. For TurboTax to properly calculate the basis for the sale it needs information about each purchase and what you're being asked to enter here is the number of shares from each purchase, (which might not be the same as the number of shares in the actual grant), that made up this sale.
Tom Young
