This refers to how many girls there are over in that place ("there"). Там is the "there" which contrasts to the "here" of здесь or тут. While the Russian version is clear, the English version of this sentence is unfortunately ambiguous, because we use "there" to mean both a literal там location and a general sense of existence.
Apostrophe is a way to write soft sign (ь) with the Latin script. Soft sign changes the pronounciation of the preceding consonant; л is «hard» l, /l/, while ль is «soft» l' sound, /lʲ/.
Compare the difference in pronounciation of some words:
In the past, ь was a very short i-like (or e-like) consonant, but it disappeared, changing the way consonants are pronounced. English doesn't have a notion of soft/hard consonants, so it might take some effort to understand them.
It wasn’t me who put it — I'm just going through the forum answering the questions (it makes me happy that people learn my native language), but the course was created by people like Shady_Arc.
Why not "How many girls?" There are many contexts in which that is perfectly natural.