Q: Il vous faut combien de litres? How many liters do you need?
A: Il me faut 50 litres. I need 50 liters.
Slip in a “me, te, lui, nous, vous, or leur” in front of il faut to indicates who (indirect object) needs X: “to me, to you, to him/her/it, to us, to you, or to them”. This is a roundaboutor way of saying I need, isn’t it? First, we start with the Il faut (One needs) and then we personalize with me-te-lui-nous-vous-leur (indirect obejct).
The indirect object is the recipient of the verbal action: Who/What needs whom/what.
Il lui faut un manteau – He/She/It needs a coat. Ask the following questions to get the hang of it : Who needs a coat? The man does. The woman does. The chihuahua does. What does the chihuahua need? A coat.
Remember that “falloir” refers to general needs, requirements and necessity (Il faut + verbe (series on falloir, pouvoir, devoir)).