There are, in different cultures, different "sacred" and "important" numbers.
Insofar as the folk-mgic of hoodoo is a blend of the African folk-magic of slaves with the folk-magics of European slave-owners and bond-servants and the folk-magic of Native Americans and the folk-magic of other small American populations, such as Jews, Chinese, and Asian immigrants, you will find in hoodoo a reflection of those cultures and their favoured numbers.
Here are some ways to think about numbers. The emphasis is on THINK, not on "follow a rule-set."
1 is lucky in and of itself; one John the Conqueror root may be carried as a "lucky piece," and although not many people think of this as a "hand" because it is a solitary item, but stiil, it may be thought of as a hand.
2 is a pair and is highly favoured the world around in making love-charms; two of any seed, nut, bud, or berry stands for the two people who will love one another in a mated pair; this is found everywhere in hoodoo as well as in most systems of folk-magic; two small Lodestones wrapped in flannel, two Balm of Gilead buds, and other pairs are often used in hoodoo, and they need no third item to make them functional as a love-hand.
3 is favoured among Jews and Christians; to the latter it symbolized the Holy Trinity; it is also found in modern Hinduism as the Trimurti; it is a common number in many forms of folk-magic, from Stregheria in hoodoo, insofar as they are basically Judeo-Christian folk-magic; it is typically thought of as the minimum number of ingredients in a hand -- with the exception of a love-hand, which may contain two items.
4 is favoured among many Native Americans and some Asians; to some Chinese-speaking Asians it means bad luck and death; it is found in hoodoo among those of Native American lineage as a number of power; in Africa, especially in the Congo, it is a sacred number for the division of space, the crossroads.
5 is, in hoodoo, the most common shorthand representation of the crossrossroads cross (+ or X in a circle), the 4 corners being defined by 5 spots, including the center; this is called a quincunx by ethnographers.
6 is favoured among some Native Americans as representative of the six directions (derived as four planar directions and two vertical directions), but these tribes are not located in in the American South-East, where hoodoo developed, so 6 is a minor number in hoodoo, except as two groups of 3.
7 is favoured among Jews, Christians, and all cultures that engage in sky-watching, as it derives from the "seven sacred planets" -- 5 visible planets plus the Sun and Moon, portrayed as deities -- from which is developed the concept of 7 days of the week, each "ruled" by a God-Planet. It is a prime number.
8 is favoured by some Chinese people and is identified with the modern infinity symbol elsewhere; not central to hoodoo, but as the first of the notable binary duplicators (in the series 1, 2, 4, 8, 16, 31, 64, etc) it stands for something that may endure for a long time.
9 is favoured among some Africans as three times three; it is less central to European folk-magic.
10 is a purely secular number, as can be seen with the decimal system, ten-based, with which skeptics and scientists have long tried to replace the older 12-based system of feet and inches.
11 is a prime number and a lucky winning number (along with 7) in the popular African American game of shooting dice.
12 is favoured among some Europeans as the number of Solar months in a year, as a very divisible counting-number, a dozen, which has many factors for its size; it is contrasted with --
13, the number of Lunar months in a year, which is disfavoured among practitioners of Solar religious inclination, but sometimes clung to by conquered people (both in Europe and the Americas) as a remnant of an older, vanquished sacred Lunar calender; hence, bad-luck that is secretly empowered as good luck, such as "Lucky 13" -- "reverse bad luck -- or the proverbial "baker's dozen" -- an extra, unexpected gift or price-discount.
There are other interesting historical and philosophical beliefs adhering to these and other numbers, such as 11, 22, 24, 99 and so forth, but these should start you thinking.
Now, the question that has not been asked is -- "How many people actually COUNT the number of ingredients in a hand?"
The answer: "A lot fewer than rule-obsessive people want to think!"
That's right -- the counting of items, the obsessive poring over of lists of herbs to make them come out to a propitious number is not central to folk magic as a whole. It has more to do with personality types than with cultural systems. In other words, in every culture, some people are "counters" and some people are not. The number favoured by any given "counter" will be culturally induced, but the desire or obsession to count is personal.
In fact, to those who are not obsessive counters, counting ingredients can seem entirely bogus, for, when Spirit says, "Grab this and this and this and this," are you gonna argue with Spirit that, "Oh, no, that's not an ODD NUMBER!"
Remember, thinking is what makes you human. Following rule-sets never lets you rise beyond the status of a well-trained dog.