I can't help thinking of "hoeveelheid" as something like "howmuchness", which is quite funny, but very logical indeed.
Why is it 'totale' rather than 'totaal' before both 'nummer' and 'hoeveelheid'? I thought the difference depended on whether the noun took 'het' or 'de', but I have just seen 'het totale nummer' and 'de totale hoeveelheid'.
You can read this in a different unit, but I would say it's because when you have the article clearly stated (be it "de" or "het"), it's always with the -e ending (so, 'totale'). This happens here when you have "HET totale nummer" or "DE totale hoeveelheid". However, it's 'in totaal'. Hope it helped!
That depends on the situation, there are different types of totals, total weight, total sum, etc. If you want to leave out hoeveelheid in Dutch you have to change the sentence to Wat is het totaal?
There are three noun genders in Dutch: masculine (mannelijk), feminine (vrouwelijk) and neuter (onzijdig). Masculine and feminine nouns take 'de' and neuter nouns take 'het'.