If I have investments from multiple companies, do I fill out different capital gains pages for each company or try to fit it all on the same capital gains page?

On the capital gains page, there is a box that says "Corporation name and share class or name of issuer" which makes it sound like you should only put one company's name in the box. So I'm wondering if we have to fill out a new page for each investment from a different company. 

Also, I'm curious as to how we would indicate a capital loss with this. Do we just input a negative number or put the number in parentheses?

Investors and rental owners

If you have shares in different companies like INTU, GOOGL, AAPL etc, then yes you will need enter a new page for each.