Step 1: Recall the definition of dacoity under the IPC.
Dacoity is essentially robbery committed jointly by a group of persons, and the law fixes a minimum headcount for the offence to be called dacoity rather than robbery.
Step 2: Recall the exact number.
Section 391 IPC says that when five or more persons conjointly commit or attempt to commit a robbery, every person so committing is said to commit dacoity.
Step 3: Confirm against the options.
Numbers like 2, 3 or 10 do not match the statutory threshold, only 5 does.
\[ \boxed{5} \]