Actually, very few ever returned back to Greece.
Alexander started out from Greece with 40,000 Men. He has to fight a punishing campaign to subdue Persia which included three major battles and countless numerous one. Even some small skirmish like
Battle of the Persian Gate resulted in 14,000 casualties. Then he has to fight tribes living in Afghanistan. By the time he reached India, majority of his army consisted of Asian levies. Here after winning battle of Hydasepes,when his army rebelled and refused to march further into India, he subdued some local tribes and then divided his army in two part and send one vis sea to Persia and marched other half through a well known desert. (Though some scholars believe that he lost Hydaspes or won it with heavy casualty, and marching through desert is a story cooked up by Greek historians to hide defeat or near defeat of their God-King).
Given that Ambhi of Taxila was his satrap, Alexander did left a small force to ensure that he remained as such. But how many, I would say given the circumstances it would be less than 1000. After this, defeat of Seleucus by Chandragupta ensured that no further Greek gene flow was possible (also Greeks were not enthusiastic to migrate to such far off land, and neither their base population was enough to populate anything). When Indo-Greek empire did have a chance to rule India for brief period after fall of Mauryan empire, they were more Indian than Greek.
