What it is
A survey number is the number the government assigns to a specific parcel when it surveys a revenue village, and it stays with that block of land regardless of who owns it. Every parcel in a village gets one within a running sequence, and it is how a plot is identified in every record that follows: patta, chitta, encumbrance certificate, and the FMB sketch. Where a survey number has been split among owners over time, each fragment carries a subdivision suffix (the /1, /2 after it) rather than a fresh number.
Why it matters when you buy
Every document you check during a purchase should point to the same survey number: the seller's patta, the encumbrance certificate, the FMB sketch, and the draft sale deed. A mismatch, even by one digit, means you may be looking at the wrong parcel or an outdated description, and it is worth stopping to resolve before you sign anything. Because the number does not change with ownership, it is also the constant you use to search records and pull certified copies for a plot you have not seen documents for yet.
Where you find it
It appears at the top of a patta-chitta printout, along with the subdivision if one exists, and again on every FMB sketch and A-register extract for that village. Sub-registrar offices index deeds by survey number too, so it is what you quote when asking for a copy of a registered document at the SRO.