You usually get a new I-94 any single time that you enter the US. However if you have to many entries with no clear purpose the Immigration authorities may deny your entry. Or if you over stay your B1 visa they can also deny your entry.
I agree with the other attorney. Also, you must have permission to enter Mexico.
Andre Olivie, Esq.
Legal disclaimer: This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice. Do not rely on this advice without speaking to an immigration attorney in detail about your case. This message does not create an attorney-client relationship.
Travel to a contiguous area for up to 30 days does not require a new B-2
visa to re-enter the U.S. The same I-94 card is used as the stay is
considered continuous. There are some exception to this rule (such as
when the alien applies for a visa while abroad).