Criminal conspiracy is unusual among offences because it punishes an agreement itself, not just the completed act the agreement was aimed at. Ordinarily, criminal law waits for some act beyond mere planning before it steps in, since thoughts and plans alone are not usually treated as crimes. Conspiracy law makes an exception, but the exception is calibrated to how serious the planned offence is.
Section 120A tells us conspiracy needs an agreement between two or more people to do something illegal, or to do something legal through illegal means. Section 120B then draws a line based on seriousness: if the object of that agreement is an offence carrying death, life imprisonment, or rigorous imprisonment for two years or more, the agreement by itself is enough for a conviction, no further step is required. For lesser offences, the law is more cautious and expects something beyond the bare agreement, typically an overt act, before conspiracy becomes punishable.
This explains why an option requiring five people, or one that lets a lone individual's private intention count as conspiracy, or one that bars charging conspiracy alongside the substantive offence, all miss the mark. Two people is the floor, an agreement toward a serious offence is punishable on its own, and conspiracy regularly stands alongside the crime it was aimed at, since they are separate offences with separate ingredients.
So the legally accurate statement is that the mere agreement is enough to constitute criminal conspiracy where the intended offence is punishable with death or rigorous imprisonment.