To me it seems impossible to assign value to the amount of MWI-copies of you, not least because there is no way you could even conceive their number, or usually make meaningful ethical decisions where you weigh their amounts.

As it happens, you totally can (it's called the Born measure, and it's the same number as what people used to think was the probabilities of different branches occurring), and agents that satisfy sane decision-theoretic criteria weight branches by their Born measure - see this paper for the details.

I would not feel like I am killing myself just because part of my future copies never get to exist, nor would I feel bad for the copies of the rest of all people - no one would ever notice anything, vast amounts of future copies of current people would wake up just like they thought they would the next morning, and carry on with their lives and aspirations.

This is a good place to strengthen intuition, since if you replace "killing myself" with "torturing myself", it's still true that none of your future selves who remain alive/untortured "would ever notice anything, vast amounts of future copies of [yourself] would wake up just like they thought they would the nloext morning, and carry on with their lives and aspirations". If you arrange for yourself to be tortured in some branches and not others, you wake up just as normal and live an ordinary, fulfilling life - but you also wake up and get tortured. Similarly, if you arrange for yourself to be killed in some branches and not others, you wake up just as normal and live an ordinary, fulfilling life - but you also get killed (which is presumably a bad thing even or especially if everybody else also dies).

One way to intuitively see that this way of thinking is going to get you in trouble is to note that your preferences, as stated, aren't continuous as a function of reality. You're saying that universes where (1-x) proportion of branches feature you being dead and x proportion of branches feature you being alive are all equally fine for all x > 0, but that a universe where you are dead with proportion 1 and alive with proportion 0 would be awful (well, you didn't actually say that, but otherwise you would be fine with killing some of your possible future selves in a classical universe). However, there is basically no difference between a universe where (1-epsilon) proportion of branches feature you being dead and epsilon proportion of branches feature you being alive, and a universe where 1 proportion of branches feature you being dead and 0 proportion of branches feature you being alive (since don't forget, MWI looks like a superposition of waves, not a collection of separate universes). This is the sort of thing which is liable to lead to crazy behaviour.