In futures for calculating no of contracts why they have divided by the lock in rate price, if I would have calculated it by 20m/1.3698 which is futures price would it be wrong?
Because we always divide by futures price only and then by contract size to calculate no of contracts.
It is arguable as to whether to use the lock-in rate or the current futures price. You would get the marks for either (and usually it ends up making no difference to the number of contracts), although using the lock-in rate is arguably better.