美国签证的visa foil number就是您的签证号，visa foil number在签证纸的右下角能找到，是一串红色的号码，字母+7位数字。
如图所示：
美国签证的visa foil number一般在登记EVUS，或续签的时候才需要提供，续签时会要求填写上一次的visa number。
以下是来自美国移民局关于visa foil number的解释：
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 Notice of Action mailed by USCIS. The 9 digit number (sometimes 8 digits since the first number is assumed to be 0) would be above the beneficiary name (A *** *** ***). If the number's not there, then there's none assigned. Depending on why you need the "visa number", you can use the receipt number as proof that there's a pending petition (the receipt number is 13 characters long and starts with three letters like EAC, WAC, or MSC). For Non-Immigrant Visa, Visa number is the RED Number on the Visa itself. This answer is from USCIS.
美国签证申请流程：
第1步：
在线完成签证电子申请表(DS-160)的填写。
第2步：
缴纳签证申请费，预约面谈。要预约面谈，需提供以下三条信息：
- 护照号码。
- 签证申请缴费收据上的编号。
- DS-160确认页上的十(10)位条形码编号。
第3步：
请按照约定的时间到美国大使馆或总领事馆进行面谈。
