Trademark Statement of Use

As the others have stated, the general rule is one specimen per class. But beware that if the classes describe a long list of goods/services, or describe "a whole line of _________" goods, the USPTO could require additional specimens for additional goods. Also, remember that when you sign the Statement of Use declaration, you will be swearing under oath that the mark is being used on all of the listed goods/services, not just one per class. While the USPTO may not check that, if you in fact are not using the mark on all of the listed goods/services a third party could attempt to cancel your resulting registration based upon that alleged inaccuracy or untruth before the USPTO.

You need to provide one per each class; for products a picture of the good (or its packaging) showing the mark on it, in case of services you need to show publicity describing the services bearing the mark (Pamphlets, websites, etc). It is very important that each specimen bears the mark as it was filed. Let me know if you need further help/information.

This is generally covered by TMEP Sections 904.01, 904.03, and 1301.04. Normally, you'll need to provide one specimen per class.