To think that ashes are just from one person?

[MacTweedy]

When someone's cremated, how do you know you're given just their ashes? Is the fire cleaned out after each body?

When pets are cremated it's almost always NOT.

Or so says Freakonomics

It is cleaned out each time. Is more like an oven than a fire. Really intense heat. A crematorium would usually have more than one 'oven' too so can empty one whilst the other is in use. Dh is 'in the business'

Yes its your deceased person. Most of the ashes are the coffin rather than the person unless a shrouded cremation has been requested.
Bits that haven't burnt well like metal handles on the coffin and metal hip replacements are not included in the ashes.

the coffin is on a tray, bit like the bottom of your toaster. its pulled ou and left to cool, then another tray is inserted for the next cremation etc. There isn't only one oven though.

Once the ashes are cooled, they are rolled/crushed to get rid of any lumps/foreign objects. This is an interesting set of photos

www.theguardian.com/society/gallery/2008/sep/02/photography