Yes, it seems like it’s four. I haven’t been able to find a English version of this rule anywhere but it’s contained in article 3 of 臺北市短期補習班管理規則 (translates roughly as “Taipei City Short-term Supplementary Learning Center (Buxiban) Management Regulations”) http://www.doe.gov.taipei/ct.asp?xItem=1211357&ctNode=63484&mp=104001. Quote from article 3: 第三條 本規則所稱補習班，指以補充國民生活知識，傳授實用技藝或輔導升學為目的，對外公開招生、收費、授課且有固定班址，預收學生人數並達五人以上之短期補習班。Basically, from my understanding, it’s defining what a Buxiban is and included in that definition it states that it would have 5 or more students already enrolled. But I think you’d need to check this article 3 with someone who has much better Chinese than I do. Hope this helps.[/quote]
“‘Buxiban’ [tutorial school] as used in these regulations refers to a short-term buxiban which has as its aim the supplementation of national livelihood through knowledge, the imparting of practical technical skills, or further (tutorial) education; is open to the public for enrollment; collects fees; has a fixed (teaching) location; and has a projected student population of at least five.”
(I’m not sure about the “national livelihood” part. Can anyone think of a better translation?)
Of course, if in doubt, check with a lawyer and/or the relevant authorities. Under the Supplementary Education Act, every city/county sets its own regulations.