When you enter the "cost or adjusted basis" for an item that you donate, you enter either the original amount that you paid for the item or you enter the original purchase price, as adjusted.
Why would the cost (or "basis") of an item get adjusted?
Perhaps you used the item in a business setting and depreciated it before donating it. Each dollar of depreciation is subtracted from the original cost or basis, hence, the basis is "adjusted".
If you made substantial improvements to the item before you donated it (as opposed to repairs and maintenance), then you increase the basis by adding the cost of the improvement to the original cost. Example: you added a new bathroom to a house before donating it to a qualified charity.
As noted, normal wear and tear on personal use items and normal repairs and maintenance do not adjust the basis, so for many donated items, the cost and the adjusted basis are the same.
I have the same question: I donate alot of clothing so what number can I use on this line?
If it is asking for the cost, include the amount you paid for the items.
If it is asking for Fair Market Value, please see this Value Guide.
If you haven't filed yet, go back to the Charitable Donations section under Deductions & Credits and choose Edit/Update.
If you have already filed and been Accepted, wait until you receive your Refund to Amend Your Return to make the corrections.
Here's more info on Do I Need to Amend?