What was the first big scale (over 50 tanks used) battle with the use of tanks in WW1?
Please note that the typical examples don't constitute good answer:
Battle of Flers-Courcelette (Somme) was the first use of tanks but definitely not >50.
Battle of Cambrai (November 1917) - which seems to be the standard answer on Google search - appears to be incorrect: as per Wiki, there were earlier battles with over 50 tanks fielded, though there were no good details/references.
Second Battle of Villers-Bretonneux, France (1918) is notable for being first tank-vs-tank battle, but it was mere 3 against 3 quantity-wise.