James has a credit card he opened at the beginning of 2017 only for business expenses. For this year, should he keep each receipt or are the monthly statements adequate?

[HelenaC]

Both receipts or monthly statements are adequate proof. When you incur the qualified expense by credit card, the IRS requires a statement that shows the transaction date, the payee's name and the amount you paid.

Acceptable proof includes, but is not limited to, cash receipts, bank statements, cancelled checks and pay stubs.  Do not send receipts with your tax return.

See the IRS article:  What kind of records should I keep? - IRS.gov