A minimum of one share must be issued upon incorporating. Additionally, if you plan on having more than one shareholder, then you must issue at least one share per shareholder.
Often, individuals who wish to be sole owners issue a single share in order to own 100% of their company. However, if they later wish to split the ownership, or change the structure of the business, having only 1 share could lead to complications down the line.
A common practice is to issue share capital which is easily divisible in the future (for example it may be best to issue 50 or 100 shares upon incorporation). By doing so this will allow you to change the ownership of shares more easily down the road. Many businesses tend to issue 100 shares of £1 each, with each share representing 1% of the business.