美國簽證的visa foil number就是您的簽證號，visa foil number在簽證紙的右下角能找到，是一串紅色的號碼，字母+7位數字。
如圖所示：
美國簽證的visa foil number一般在登記EVUS，或續簽的時候才需要提供，續簽時會要求填寫上一次的visa number。
以下是來自美國移民局關於visa foil number的解釋：
The visa number can be found on the Visa sticker on your passport page. The number can be 8 or more characters long, and in RED color, located towards the bottom right hand quarter of the Visa sticker. Be advised that the "Control Number" is not the Visa number.
If you mean a non-immigrant visa such as a B1, B2, F1, H1B, then it's on the visa itself. The visa would be the nice-looking laminated sticker (the new ones, at least) on your passport, and the number would be a long number on the top of the visa. If you lost it, there's going to be some minor problems obtaining the number, but you would have to contac the Dept. of State or the consular section where you obtained the visa. If you're talking about an immigrant visa, an A-number may or may not be assigned to you if you're still waiting for the petition to be approved. Generally, this would not be termed a "visa number". Check out any I-797