Obviously we don't have precise information for the makeup of Alexander's army at any point, but there certainly would have been hundreds if not thousands of men in India that had initially set out from Greece/Macedonia with him. The army suffered casualties throughout its campaigning, of course, but it is important to remember that due to the nature of Alexander's campaigns he was able to secure victory without losing lots of men to attrition. Alexander, and his father Philip II before him, were firm believers in the idea of a decisive battle, so when they chose to fight they threw everything at it in an attempt to secure an ultimate victory. Even when initially rebuked, both Alexander and Philip II soon returned with overwhelming force to defeat their opponents.

The acts of brutality that Alexander implemented in some of his campaigns (notably against Tyre and, later, in the Indus Valley) were attempts to crush the morale of neighboring cities and settlements so that they would be completely cowed. A large portion of the massacres are somewhat blamed on the frustration of some of his soldiers, at least according to Ian Worthington and one of his brilliant graduate students (I was fortunate to learn from them). At the very least, many of the officers and generals would have been soldiers that left Greece/Macedonia with Alexander. It is important to mention, however, that Alexander regularly requested reinforcements from both Macedonia and the Greek city-states that were, effectively, his vassals. Even before Alexander died there were problems of former soldiers that had served with him running amok throughout his newly-conquered empire (mainly Mesopotamia and Anatolia).

Alexander the Great: A Reader by Ian Worthington

A Companion to Ancient Macedonia by Roisman and Worthington

As a follow up question, how would these soldiers have kept or transferred their spoils of war? Was there a secure way to send money back home?