Manglik dosha — also Mangal dosha or Kuja dosha — is the condition of having Mars in certain houses. It is the most socially consequential idea in popular Indian astrology, and the arithmetic behind it deserves to be widely known.
The commonest definition counts Mars in the 1st, 4th, 7th, 8th or 12th house from the ascendant. That is five houses out of twelve. If Mars were distributed evenly, about 42% of all people would be Manglik. Real distributions differ a little, but the order of magnitude is right: this is not a rare affliction, it is a description that fits something close to two people in five.
Schools also disagree on the definition itself. Some count from the Moon or from the ascendant of the Navamsha rather than from the birth ascendant; some include the 2nd house; some exclude the 1st. Under one reading a person is Manglik and under another they are not, with no way to adjudicate between them. Classical texts additionally list numerous cancellations — Mars in its own sign, in Aries or Scorpio, aspected by Jupiter, both partners Manglik — and a reading that pronounces the dosha without checking them has not finished the job even by traditional standards.
What this site will not do is treat the result as a decision. Manglik status is used in India to break off engagements, to justify ritual marriage to a tree or a pot, and to price people in the marriage market. A configuration shared by two-fifths of the population, defined differently by different schools and cancelled by a long list of conditions, is not a sound basis for any of that. No dosha should decide a marriage.