Booster will add extra absorbancy if you LO is a heavy wetter.
They aren't generally needed on an newborn because you are changing them frequently anyway.
Once they stop giving you dirty nappies overnight - then a booster would be helpful to turn your fitted nappies into a longer lasting 'night nappy' which would then last all night.
Fleece liners add the stay dry layer to the inside of the nappy.
My LO isn't bothered by a wet bottom & its not making her sore - so Im skipping these at the moment (a personal preference of mine) but others use them all the time, so worth having some in. Also if they do get some nappy rash & you need to put cream on - then they protect the nappy from the cream!
They dry super quick - so probably about 10 is enough.
They really come into their own one weaning starts - allows you to drop poop down the loo before washing nappies.
Newborn poop is runny - so just soaks through them.
Hope that helps!