Pretty much, yes.
Singular πόσος, πόση, πόσο (depending on the gender of the thing you are counting) is "how much", plural πόσοι, πόσες, πόσα (again depending on the gender of the thing you are counting) is "how many".
So πόσος χυμός, πόση μπύρα, πόσο νερό "how much juice, how much beer, how much water" and πόσοι άντρες, πόσες γυναίκες, πόσα παιδία "how many men, how many women, how many children".
Greek has no "sh" sound, so there is no pressure to keep "s" and "sh" sounds separate lest they be confused.
This means that the "s" sound has a bit more "room to spread out" in Greek, and you might hear a pronunciation that is a bit further forward than an English "s", in the direction of English "sh".
It's not wrong, just different.
If you're unsure of how to make the sound yourself, stick with English "s" (a full-blown English "sh" would be too far forward, I think), but be prepared to hear some intermediate versions from native speakers.
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