When incorporating a private company limited by shares, you must issue at least one share when incorporating.
If you intend to have more than one shareholder, you must issue at least one share per shareholder meaning that if you have two shareholders when you incorporate you must have 2 shares. You cannot divide a whole share into parts.
You might want to issue an easily divisible number, such as 50 or 100, when you set up to avoid difficulties down the line. One person can own all 50 or 100 shares but by creating more than one share you have the flexibility to issue shares in the future without complication.
After you have decided on the number of shares you also need to give those shares a value. Typically, companies will issue a nominal value to the shares of £1- this way the share is easy to pay for and does not carry a high financial burden for the shareholder. If you create a considerable number of shares upon incorporation you might consider selecting a nominal value of 1p.