I was reading about warship construction in the days of sail and came across an interesting way of estimating the largest possible number of Numenoreans surviving the Downfall in company with Elendil.

First the Numenoreans were great ship-builders and they built their ships from wood - deforestation was one of their points of conflict with other nations in Middle Earth (see Lond Daer in U. Tales).

Wooden ships have an inbuilt size limitation beyond which "Hogging" (the ends of a ship drooping relative to the midship) becomes excessive and results in appalling sailing characteristics. In the age of Nelson the largest ships in the world were the great three-decker warships, such as the Victory of 100 guns. These ships, particularly the 120 gun vessels such as the French Commerce de Marseilles or the British Caledonian, were reaching the upper size limit for wooden construction. Indeed the largest vessel in the world, the Spanish Santissima Trinidad of 130 guns, had perhaps exceeded the limits as she was noted to be a poor sailer and was on the point of being relegated to harbour duties when she was destroyed at Trafalgar.
These ships had a crew of around 1000 men, and they were very tightly packed together.

Returning to the Numenoreans, we can assume that they would be capable of building ships up to this size limitation and as their ships had to be capable of ocean voyages they would not have attempted to build beyond the limitations as their sailing abilities would have been badly compromised.

Elendil had 9 ships saved from the Akkalabeth ("Tall ships three times three"), and allowing for a greater packing in an emergency of 50%, the greatest possible number of Numenoreans would be around 13,500. Possibly less as Numenoreans would have been physically larger than 18th century seamen, though this could be compensated by the presence of women and children amongst the refugees.