Ok, so I tried to make the question more precise and then to work out the answer using an hypergeometric calculator (
https://aetherhub.com/Apps/HyperGeometric). Maybe you can tell me if the math checks out.
Let's assume that
Priest of Titania is in my opening hand, and that I have N other Elves in my deck (for example we can assume N=10). I'll also assume that all these Elves have CMC=3 or less, so that I can play them on turn 3.
I play
Priest of Titania on turn 2. What is the probability P that I will able to tap her for more than 1 mana during turn 3? Or, in other words, what is the probability that I draw at least another Elf before turn 4?
Related question: How large must be N to make P=0.90?
So, my reasoning is the following: I assume one of the cards in my opening hand is the Priest, and in addition I have my commander in the command zone. So my sample size is 98 cards.
On T1, I see 8 of these cards, of which 1 is the Priest. So, I see 7 of them. If N=10, the probability that one of those is an Elf should be P=54.1%. On T2, I will have seen 8 total, etc., so that:
T1: P=54.1% (cards seen: 7+1)
T2: P=59.2% (cards seen: 8+1)
T3: P=
63.7% (cards seen: 9+1)
This should be the probability to tap the Priest for more than 1 on turn 3.
I've also made some trials and found that I need 21 other Elves with CMC less than or equal to 3 to have P=0.90 (0.897 to be precise).
Is this correct?