system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do companies determine how many shares their company is made up of?

My knowledge of this rusty at best, so please feel free to correct any mistakes on my part. I've often heard the term 'X owns 10 shares in company Y'. How do companies determine how many shares their company is made up of? Is this something the company agrees on, or is this something standardized by governments? Like, if I own a company, can I decide to have just as many or as little shares I want my company to have?
gpt
Shares are a simple percentage of ownership.  The number of shares owned out of the total number of shares;  so the number of shares alone is not relevant information until it is compared to the total shares and you calculate a percentage of ownership.

Also, the number of shares can be changed by a vote of the Board... Stock splits and reverse stock splits, for example.   

Some companies want wide share availability, so they adjust total shares to make the individual share price "reachable" for all investors (Apple is currently in the $120/share range because they have many, many total shares now) whereas other companies prefer not to deal with millions of investors -- which is why Berkshire Hathaway A shares are about $203,000 per share.  As you can see, BH share price is much higher than Apple, yet Apple is a more valuable company because it has many more shares at that lower share price. 

Each share of Apple is a tiny, tiny fraction of ownership, each share of Berkshire Hathaway is a much larger percentage of ownership.

If Apple had kept its original number of shares fixed, each share would now be very high, like Berkshire Hathaway.

Another way to think of it is this:  If Apple was owned by just ten people equally, they could have all the ownership in one share each; each of those 10 shares would be worth about $67 Billion.  But that would make it very hard for any of them to sell any portion of their ownership.