It rather depends on how much meat is left on it

LOL - that doesn't sound especially helpful, but the fact is that it varies widely.
At 'worst' a chicken back has almost no meat at all (in which case, it provides the bone portion of a meal, and you need to add meat to have the meat portion). At best, chicken backs can be quite meaty. Some even come with the offal attached too.
I'm afraid you've just got to eyeball the back in question and make your own approximation. If it doesn't look as though there's an equal proportion of meat to bone there, then you need to add meat (in the same meal, or in the next) to compensate.