Usage of „Erhöhung bei der Anzahl“

maybe the example with money is more common... "you had 3 dollar now you have 4." your money didnt increase.(because it cant, its not specified how exactly its meant...) the amount of money you have did increase.

its just like in programming...
or anything rly:
"a human cant increase", but the human has height, age, fingernail lenght hairlength and so on, and so on. these properties can increase.

Sehr geehrte Damen und Herren, ich melde mich in diesem Gremium zu Wort, um die Erhöhung bei der Anzahl der Kopfläuse in unseren Schulen hervorzuheben.

but that sentence itself wouldnt be commonly used. better would be:

Sehr geehrte Damen und Herren, ich melde mich in diesem Gremium zu Wort, um das vermehrte Auftreten von Kopfläusen an unseren Schulen hervorzuheben.

and it also goes around the "anzahl" thing :D also "an" is better than "in" in those contexts.