27 CFR §24.46

1. (a)An employer identification number will be assigned pursuant to application on Internal Revenue Service (IRS) Form SS-4 filed by the taxpayer. IRS Form SS-4 may be obtained from the director of the service center or from any district director.
2. (b)An application on IRS Form SS-4 will be made by the taxpayer who, prior to filing the first return, has neither secured nor made application for an employer identification number. An application on IRS Form SS-4 will be filed on or before the seventh day after the date on which the first return is filed.
3. (c)Each taxpayer shall make application for and be assigned only one employer identification number, regardless of the number of places of business for which the taxpayer is required to file a tax return under the provisions of this part. (Pub. L. 87-397, 75 Stat. 828, as amended (26 U.S.C. 6109))