You can appoint as many Executors and Trustees as you like within your Will but when the time comes a maximum of only four can act.
You can appoint one Executor acting solely but it is recommended to appoint alternative Executors in case your first choice decides not to take the position or dies before you.
With Trustees, it is generally recommended that a minimum of two are appointed to act jointly; this is due to legislation relating to land transactions including the sale of property.