All these words are contextual, and therefore there's a degree of vagueness to them. 'A couple' usually means 2 or about 2, and you use it when you either don't know exactly how many or want to be deliberately vague.
If I said 'Get a couple of tins of tomatoes' I would expect 2 or 3, because if I had definitely wanted 2 I would have said 2.
If I go to the pub with a large group of people, and I say later 'We had a couple of drinks', I don't mean that every one of the group had exactly 2 drinks - I mean that most of us probably had 2 drinks (and most likely I did), but some people might have only had 1 and some might have had 3 or more.
If I say I've met someone 'a couple of times' and if that's over a lifetime, then I know that I've met them more than once and probably fewer than 5 or so times, but I don't know exactly how many.
'Few' is always more than 2, but as others have said, it implies scarcity or not much of what there is or was. 'A few biscuits left' - if you started with 100, it might be as many as 10, but if you had 10 originally, probably 3 or 4.
'Several' is more than 'a few' if talking about small numbers, but actually could be fewer than 'a few' if you're talking about large groups ie 'a few of the million refugees' could be as many as 1000, and is more than 'several' which is generally under 100.