There is no limit to the amount of shares you have issued into your company. You need to have at least one when creating a Limited by Shares company. There are different factors to consider such as the amount of capital needed to be raised, whether there is enough "cash on hand" to cover the amount of capital issued and what stipulations in regards to decision making is listed within the articles of association for the company.
The default value and amount of shares issued upon incorporation is 1 share at 1 GBP, but this is entirely up to the company.
If unsure as to how you would like to structure your business, we recommend speaking to a business advisor.