Initially USCIS will probably give you some deference and may approve a new office L fro 1 year, even if you have not yet hired any employees. By the end of the first year USCIS will want to see that the business has become more than just a job for you. So yes you should have at least 1 other employee by then to maximize your chance of getting the visa renewed. Additionally, after the company has been up and running for at least a year you could potentially apply for a green card as a multi-national executive. Both the immigrant and non-immigrant visas require that you keep the overseas company up and running.
L-1A cases are fairly complicated for new business start-ups. Obviously you wont' have any employees until the U.S. company can be created, but your business plan has to show how you are going to be a manager or executive of the enterprise. That usually requires having employees to do the front line work of the company. You need to consult with an immigration attorney who has experience in L-1A start-ups and dealing with the sometimes onerous requirements of USCIS in adjudicating these cases.
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Discuss your options privately with an immigration attorney. A "new office" L is similar to an E, but you'll want someone to go through the different visas with you in much more detail than can be done here on AVVO. With either, you'll be making a huge investment, and you should not proceed on the basis of self-help or Internet research. You'll end up losing everything.
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I agree with both my colleagues
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.