When in the Supreme Court, I asked my colleague, Justice H.S. Bedi, who is a direct descendant of Guru Nanak, why it is said that one Sikh is equal to 125, 000 ( sava lakh ) soldiers' ?
He said that there is a story behind it
When Guru Gobind Singh (the 10th Sikh Guru) fought the battle of Chamkaur (a place in Punjab) against the Mughals. 40 valiant Sikhs confronted the 10 lakh mughal soldiers in the chilling cold of December 1704.
Hence... the quote 'Sava lakh se ek ladau, tabhi Gobind Singh naam kahau', meaning 'My one sikh can fight 1.25 lakh, hence I call myself Gobind Singh'.
You can read more about this battle: Battle of Chamkaur online