The S1E1 intro to The Umbrella Academy clearly states that 43 children were born on that day as "immaculate conceptions"(or however you want to define it).
Hargreaves gets 7, but we never hear what happened to the other 36.
Did he try and purchase them and was rebuffed?
Did he know they were not "special" and not bother?
On a related note; are Hazel and Cha-Cha from the 36? They seem to be able to survive incidents that would kill a regular human.