Symbols, idols, and more all help people with faith. The more significant the thing is, the more powerfully it reflects the state of the Warp in the Materium.
Example: The 8 pointed Star of Chaos is extremely powerful because of a number of factors: 8 being a Chaos number as well as having meaning to the physical laws of the universe, the star being a Chaos symbol, and the various iconography within the star all having their own meanings, etc.
Another example: Paper seals are extremely powerful because of a number of factors as well: they are intrinsic to human ritual across the ages, the writing on them are part of the Imperial faith, the binding agents allow for more meaning to be added as part of the "glue," etc.
Also, Adam already has a divine number: 2
The Aquila has 2 heads, Humanity is a duality (good/evil, male/female, young/old, etc), the 2 aspects of Adam's divinity (God-Emperor and Omnissiah), etc.
Another thing about numerology: when you're talking about numbers as symbols, you should think about them not in the sense of what those numbers mean in a particular religion or way of thought (because that's bullshit), but in terms of concrete things. Like in the example I gave about Adam's divine number, 2 is powerful because not only does it embody all of Humanity's humanity, but because it is also highly relevant in the physical laws of the universe.
HOWEVER, Adam did not decide that 2 will be his number. It just became a part of Imperial faith during the intervening years by the various Ecclesiarchs to try and counter Chaos.