When building your deck, you often want to have at least one card with 1 or 2 mana cost in your starting hand.
Let's say we are going second, meaning we get 4 cards from our deck (first player gets 3 cards). Our goal chance is >80%, just because it's a nice big number, you pick your goals based on your playstyle and situation. Calculator shows this can be achieved with a total of 8 1-or-2-manacost cards.
Notice I set "Cards left in deck" to 25, because on your first turn you are going to draw one more card in addition to starting 4 cards.
We can use a little trick here: when you are presented your 4 starting cards, you are free to replace any of them. Replacing cards in starting hand works this way: you draw your new cards, and after that you put your old cards back to the deck, each to random place. So it is possible to draw one of your replaced cards on your first turn, but it's not possible to get your replaced card as replacement (but it can still happen if you play more than one of that card in your deck).
Basically, this means that for our initial task of having at least one low-mana drop, we can set "cards left in deck" as low as 22. We will draw one more card on first turn, but since now we are going through cards that was already used to calculate probability against, number that is shown for "21 card left in deck" will be a little bit higher than actual percentage. But you generally can disregard that difference.