Are you only referring to soldiers during the East India Company's rule of India, or during the entire history of the British Raj? Indian soldiers played a massive role in both World Wars. 850,000 Indian soldiers left the subcontinent during the First World War, fighting in France and Flanders until mid-1915, where 7,000 of them died, and also at Gallipoli (1,700 casualties) and in Mesopotamia, the theater which involved the Indian Army most heavily. Losses to Indian troops in Mesopotamia totaled over 29,000. They also fought in East Africa.

Indians also played a vital role in Entente logistics and production during the Great War, making more than 555 million bullets and over a million shells, and at least 55,000 Indian laborers served in the Indian Labor Corps, as "butchers, bakers, carpenters, shoemakers, tailors, and washermen", etc.

During the Second World War Indians played a similarly important part. Elements of the Indian Army fought in North Africa, in Italy, and most importantly, during the Burma-India campaign. By the end of the war, the Indian Army numbered over 2.5 million men, the largest all-volunteer army in history. Hopefully someone can shed a little more light on the specifics.

Source - Martin Gilbert, The First World War: A Complete History