The Khalsa is not a denomination within Sikhism. It is the initiated body at the centre of it, and the distinction it draws is between initiated and uninitiated Sikhs rather than between churches.
Guru Gobind Singh, the tenth Guru, instituted it at Anandpur at Vaisakhi in 1699. The traditional account has him ask the assembly for a head, and five men come forward in turn; these are the Panj Piare, the five beloved, and he initiated them with sweetened water stirred with a double-edged sword, then asked them to initiate him in turn. That reversal is the point of the story. The Guru placed himself under the discipline he had just created.
Initiation, amrit sanchar, commits a Sikh to the rahit, the code of conduct, and to the five articles of faith: kes, uncut hair; kangha, a comb; kara, a steel bracelet; kachhera, a cotton undergarment; and kirpan, a blade. Initiated men take the name Singh and women Kaur, replacing surnames that carried caste.
An initiated Sikh is Amritdhari. A Sikh who keeps uncut hair without having taken initiation is Keshdhari. One who follows the teaching without either is Sahajdhari. All three are Sikhs. There is no reliable figure for what proportion of Sikhs are initiated, and estimates offered for it should be treated as guesses.
The Khalsa was created in a particular situation. Sikhs were under armed pressure from the Mughal state, two Gurus had been executed, and what Gobind Singh instituted was a community recognisable on sight and unable to conceal itself. The visibility of the discipline is part of what the discipline is.