In English, owning a stake in a company is called equity.
Alternatively, you may use stake (6a) as I used before.
The equity of a business is decided by the number of shares one owns.
Or
I have a 33.3% stake in young but upcoming company.
EDIT:
Shares is also a very likely candidate. It is only applicable to (private or public) companies. A partnership or other business entity won't have shares, and any ownership may need to be described by using one of my previous suggestions. In all likeliehood, shares is the word you need in this case, and so, props to @snailboat for jolting me awake on a Sunday afternoon.