You are free to appoint as many as you choose. However, a Grant of Probate will only issue to three Executors at most.You only need to appoint one executor in your will for it to be valid. Frequently, two Executors are appointed.
There are no obligations on who the Executor should be. It should be a responsible and trustworthy adult. The person can be related to you. Most commonly spouses and close relatives act as Executors.
The role of an Executor is a gratuitous role. No payment is made from the estate to the Executors for their time, unless it is expressly provided for in the Will.
You may wish to appoint your solicitor or accountant to be your executor. If so, a charging clause will be inserted into the will and this Executor then cannot witness the Will, as this will invalidate the charging clause.
A non-professional Executor may be a witness to a Will. An Executor does not need to be present when you are making the Will.
You do not need to formally notify the Executor before your Will is executed. However, it is advisable to notify them of your intentions in advance. You should also inform your Executor where your original Will is located.