The Mungiki is not necessary a "gang" in its normal definition
A group of criminals or hoodlums who band together for mutual protection and profit
As they are heavily politically involved, and in addition have moral/religious intentions:
"Morality," he said, sipping his beer. "That is what our movement must bring back, morality. And we want to push back the sources of inequality."
Soon the conversation degenerated into crude, quasi-racist descriptions of non-Kikuyu communities and ended with a rant against homosexuality, divorce and other features of modern Kenyan society.
"We are about empowering people," said Buili, glassy-eyed. "Making people have morals, and so many other things."
(from the WP article)
They do seem to be involved in beheading of opponents, and with rituals that involve blood and/or human mutilation and sacrifice. However, it should be noted that the reports are by western reliable sources, but are given as local police statements, a police that fights the organization, and might be inclined to over emphasis the more gruesome sides of that group.
The Mungiki behead a policeman:
Police in Kenya say one of their officers has been beheaded by members of the illegal Mungiki sect.
A 2 year old boy was mutilated in a Mugiki ritual:
A TWO-year-old boy was beheaded and chopped up in a Kenyan capital slum today, police said, amid a fierce crackdown on an illegal sect blamed for a string of murders and decapitations.
The boy's mutilated torso was discovered in a maize farm and his head 500m away at a river bank in capital's Nairobi's crime-prone Korogocho slums, police commander Paul Ruto said.
The remains had no limbs, the chest was lacerated and the genitals chopped off, raising speculation that the body parts might be used in rites by the politically-linked Mungiki sect.
From the NYT article:
These days Charity Bokindo, the district commissioner of Nairobi North, is taking no chances.
Wherever she goes, she carries not one but two pistols, and she always travels with armed guards.
“The Mungiki,” she whispered, “they threatened to circumcise me.”
Police officials say the Mungiki aim to destabilize the country before the presidential elections in December and blame them for some downright ugly acts: chopping off legs, skinning heads and guzzling jerrycans of human blood. Government officials accuse them of running an extortion empire and hacking up victims as a scare tactic.
Even though he had little bad to say about the Mungiki, Dominick declined to give his last name because, he said, “these guys drink blood. You never know what they might do to you.”
As to the number of members, from the NYT article, a statement from one of the leaders:
By the late 1990s, the Mungiki went urban, Mr. Waruinge explained, taking over the city’s minibus trade. Then they diversified into garbage collection, building materials and eventually the protection racket.
“It was beautiful,” Mr. Waruinge said. “We had 500,000 members and millions of shillings coming in every day.”
But then the Mungiki made a mistake, Mr. Waruinge said, and dabbled in politics, supporting losing candidates in the elections of 2002 and falling on the wrong side of the government.
So while it is possible (from self proclamation) that the Mungiki were 500,000 men strong, they have lost power since then, and now count substantially less members.
While I couldn't find any sources about severed heads on sticks, they do probably perform some sort of rituals that include blood and/or human body.
They count thousands of members, and may have at some time had around 500,000 members.
They have a moral/religious agenda that includes at least at one occasion forced female circumcision.
They are involved in local politics.
They have a racial agenda.