It is up to the company to issue either paid or unpaid shares. A limited company would usually issue “fully paid” ordinary shares. This means the shareholder pays the full amount agreed and has no further financial obligation towards the company.
A limited company can also issue nil or partly paid shares and agree if or when the shares should be paid for. This could for example be done by settlements at set dates.
If the company was to go into liquidation, all shareholders would have to pay the amount owned for their shares in full.