I looked for an answer to this question last night and could not find a satisfactory answer, despite poking through virtually all the primary sources. I think in this case, the answer is "we don't really know exactly." We know he left 10,000 infantry and some cavalry in Bactria when he departed on the campaign in *326, but it seems impossible to tell if the expedition force had been bolstered by recruits from his eastern conquests. Specific numbers for Alexander's armies are a bit of a nightmare due to generally poor sources and his propensity to leave little pockets of armed force along his trail for founding cities, garrisons, etc. The only specific numbers I could find for the campaign are those mentioned by swammeyjoe for Hydapses, and even those are questionable.

edit: 326, not 426. In 426 the Macedonians were still a bunch of filthy barbarians preoccupied with their pretty horses while the Spartans and Athenians slugged it out.

According to Arrian Book 5 Section 14, Alexander himself personally commanded the infantry at the Battle of the Hydapses, "numbering as it did not much under 6000 men".

One section earlier you get a description of the arrangement of the infantry (Shield Bearers, Royal Guard, and then Regular Guard which I take to be phalanx, with Archers and Javelinmen on the flanks).