: : We know that there were trees in Middle-Earth unknown to modern-day shipwrights, eg. Mallorn.

: : It is possible that wood with a greater tensile strength than oak was used by the Numenoreans, thus allowing them to build larger, stronger vessels. In this case, the number of refugees could have been more than your estimate. But all very interesting nontheless.

: : Does Tolkien mention anywhere exactly what wood was used in the construction of Numenorean ships?

: Maybe the Numenoreans had ways of strengthening the materials they made their ships from. They had a lot of instruction from Eonwe, maybe not in shipbuilding, but the Numenorean power is not in question. They did after all became the greatest sea-faring nation in Middle Earth.

: Also to take things really in depth - does 13,500 individuals (if that is an accurate enough figure, maybe we should be looking at a more conservative figure) give a large enough gene pool to maintain the health of the race?

: Padster

I imagine that most of the Faithful amongst the Numenoreans had already migrated to Middle Earth, by the time of the Downfall. Probably, most of the Numenoreans living in Gondor in the Third Age would have been their descendants. Pelargir is mentioned as already being a stronghold of the Faithful in the Second Age, and no doubt they were to be found in other Numenorean settlements on the coasts of Middle Earth.

I should think that the refugees who left with Elendil would have been only the few Faithful who remained, probably no more than a few thousand.