"How many sisters do you have?"
Translation:Quot sorores habes?
For the present indicative active (what you are most likely to see currently on Duolingo):
(ego) habeo -> "I have"
(tu) habes -> "You (singular) have"
(is/ea/id) habet -> "He/she/it has"
(nos) habemus -> "We have"
(vos) habetis -> "You (plural) have"
(ei or ii/eae/ea) habent -> "They have"
You can see all the other forms on several websites. Wiktionary has a conjugation table for verbs normally: https://en.wiktionary.org/wiki/habeo#Conjugation. Wiktionary has the macrons as well.
Just be warned they are large as there are a lot of forms (many of which you won't need to know for the current Latin Duolingo course).
Yes, the -tis is often the 2nd person plural (y'all) and -nt is often the 3rd person plural (they).
The need/purpose is that they are different in person. -tis is directed at multiple listeners and -nt for a group who is neither the listener nor the speaker.
There maybe a trick out there but you will likely memorize the ending as you work with them over time.
Yes, it is wrong. Quot means "how many". You cannot translate litterally "how many", which in any case it would not be Quot multæ, but rather Quomodo multæ, but no one said like this in Latin. Unfortunately, English needs two words, where other languages only need one word; for instance, in my native language, Spanish, it is cuántas.