A couple means two of something. A few means three of something. If they say they have a couple of something then I would think that they have two of whatever it is. If they say that have a few then I would think that they have three of whatever it is.
Some and several are fairly vague but they could mean between 2 and 5. A number is also fairly vague but I think it would be 5 -10 and a good number would be 10+ in my opinion. A group would probably be 2-6 but I'm not sure.
Yes, "a couple" does mean exactly 2. However, "a few" is not tied down to an exact number.
The term "a few" basically translates to "less than the most relevant or typical 10s place or 10s multiple for whatever is being discussed."
So, if I were to say I picked up a few rocks, you can't really hold that many rocks. So, it probably means less than 10.
In a children's song, there is talk about animals eating peanuts, and there's a line "the elephants will eat quite a few." This is probably talking about less than 50 or so.
If you are talking about the scattered few among the believers in a dead religion, you are probably talking less than 1,000. (That's quite likely the largest number you could ever stretch the term "a few" to mean, but it is still perfectly applicable in that exact situation.)
The overwhelming majority of uses of this term will refer to less than 10, but there are the uncommon scenarios where it could be referring to quite a bit more depending on the context, and it is usually in terms of the most appropriate 10s place or multiple of 10 you would find relevant to whatever's being described.