Basic principle: Anybody logged in can edit anything.
Exceptions:

• If you want to edit a page without being disturbed, just write a note to that effect at the top of the page and save that change first.
  
• If two people edit a page simultaneously, the first can save normally, but the second will get a warning and should follow the directions to merge their changes with the already saved data.
  
• Personal Homepages: normally only changed by their owner, but you may append messages at the end of the page
  

In general, do follow any hints given on a page, there is generally no enforced limit, but you should not blindly ignore other people's wishes.