Per @Pheonix2105's answer, in some contexts there might be an implied to [do something to/with] after the word "left". But I don't see that as particularly relevant to the general case. For example...
1: Tom has 10 apples. He gives 3 to Dick. How many are left?
We neither know nor care whether Tom's 7 apples are "to eat", "to give to Harry", or whatever. All that matters is they're remaining (not given away, eaten, or otherwise removed from the original 10 apples).
In such contexts, left is simply an "adjectival" use of the past participle of the verb to leave...
2: Tom has 10 apples. Dick takes 3. How many does he leave?
Note that the last sentence there could just as well have been "How many does this leave?" - it's not really important whether we think of Dick as the "subject" who "leaves" (doesn't take) the leftover apples. If we use this, it's just a demonstrative pronoun meaning this action which has just been described.
If you're interested, see here for some interesting information about "Participle Adjectives" (such as "interested" and "interesting" as used earlier in this sentence).
Note that there are certain limitations on how adjectival left = remaining can be used...
3: Tom sold 3 of his 10 tickets. How many are left unsold? (either left or unsold could be omitted)
4: There are 7 left
5: There are 7 unsold
6: There are 7 tickets left
7: There are 7 tickets unsold
8: There are 7 unsold tickets
9: *There are 7 left tickets (idiomatically unacceptable)
...where #9 is "unacceptable" even though the basic construction is okay with alternatives which are "synonymous" in context (such as unsold, remaining). Because we're more used to adjectival left before a noun meaning "levo" (on the left, not the right side), we would normally express #9 as...
10: There are 7 leftover tickets (or left-over)