Is there a minimum number of employees that must be affected before a transfer under TUPE can take place?
No, case law suggests that activities involving just one employee can fall within TUPE. In Schmidt v Spar-und Leihkasse der Früheren Ämter Bordesholm, Kiel und Cronshagen  IRLR 302 ECJ, the European Court of Justice held that the Acquired Rights Directive (77/187/EEC) (now the Acquired Rights Directive (2001/23/EC)) applied regardless of the number of employees involved. In this case the employee was employed by a bank to clean one of its branches. The bank decided to transfer the cleaning of the branch to the firm responsible for cleaning most of its other branches and the employee was dismissed. The Court held the transfer of the cleaning operation, despite the fact that it was an ancillary activity and only one employee was involved, came within the scope of the Directive.
More recently, in Dudley Bower Building Services Ltd v Lowe and others  IRLR 260 EAT the Employment Appeal Tribunal held that, in principle, there is no reason why the work performed by just one employee should not be regarded as a stable economic entity so as to be capable of a TUPE transfer.