It is very complicated and is a matter of negotiation between the issuer of the shares and the buyer.
Forget the number of shares, that is not really relevant.
What percentage of the company will you be buying? Will you get 10%, 20% or 33%?
To determine the price, you have to first determine the profitability. Will the company be making profits? A formula has to be agreed for the salaries of the directors and employees as otherwise, they could declare the entire profits as salaries and so you would own shares in an unprofitable company.
What are the growth prospects of the company?
Why do they need finance?
In general, it is not a good idea to buy shares in a privately owned company. You have few rights and it is very expensive to enforce those rights in the High Court.
You will find more
here , but as Clubman says, speak to an accountant before you do anything.
Brendan