Is a Minyan required for a Brit Mila/circumcision, funeral/burial, or for a wedding/Chuppa?
Usually they tend to be done in the presence of a Minyan but is it an obligation and are there any Halachic sources on this topic?
If, by "funeral" you mean the "ceremony" done in a funeral chapel prior to burial, you should know that this is absolutely optional. Many people forego this "ritual" and have the "ceremony" (if that's the correct term, here,) at the grave.
As for burial, itself, I have attended numerous funerals all attended by at least one Orthodox rabbi who supervised the burial, yet there was no minyan. I haven't seen or heard of any opinion stating that the burial cannot occur unless there's a minyan. Of course, Kaddish cannot be recited at the gravesite unless there is a minyan.
As I have heard from a few rabbis from whom I have asked regarding a brit...
Circumcision should occur on the 8th day of the male child's life. Assuming the child is healthy for that. If not, as soon as possible after that. This is the halacha. Here too, there is no requirement that a minyan be present for the circumcision.
I have heard of numerous children that were circumcised in the hospital or in the privacy of the parents' home. The baby was healthy enough to be circumcised but needed to be isolated from other people as much as possible.
Also, a few converts told me that their adult circumcision occurred in a hospital with just the mohel and someone administering local anesthetic. No minyan.
Two rabbis told me that they married a few couples in their office. I have to contact one of them, because I understood that there was no chuppa involved. That would imply that a chuppa is optional. However, what was clear was that there was no minyan, and all that were necessary were witnesses for the ketubah and kiddushin.
Of course not, neither Mitzvah requires the presence of a Minyan, neither is postponed if no Minyan is present.
However, Seudas Mitzvah (following Mitzvah meal) after the Bris or the wedding (or Bar Mitzvah etc) requires the presence of a Minyan, and otherwise, it should be postponed.
A minyan is required on a funeral only to say Kaddish, but the Kaddish is not obligatory.