Your single entry Non B visa will give you 90 days on entry. When that’s winding down you’ll need paperwork from your employer to get a year extension from it. The paperwork needed is generally extensive so it would be best to get your employer to liaise with your immigration office as to what’s needed.
As an example here is a list of requirements for the Phuket office. Other offices will probably differ.
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To make this visa/extension multiple entry you buy a re-entry permit for 3,800 baht (a single entry is 1,000 baht). It will last as long as your current date stamp. So if you get it with the original visa that will be for 90 days max. If you buy it when on the year extension it will last max one year.