The Bible is not a calculator and it's a mistake to use genealogies as one. There were enough people to split into groups with their own languages, however many were in a group it doesn't say.
At this time people still married within their own family so they did not yet have a need to find partners they were not closely related too. They were still marrying into close family by the time of Abraham some years past this as he married his half sister Sarah.
135+ languages families have been identified across the world not 7000. Like stories, languages change and morph between groups of people as time goes on to become individualized to an area, the individualization wasn't there the day it occurred.
If the story was metaphorical God would have said so. Jesus was always clear his listeners knew when he was speaking a parable. The New Testament also confirms that Noah was a real person who experienced this event.
Luke 17:27
They were eating and drinking and marrying and being given in marriage, until the day when Noah entered the ark, and the flood came and destroyed them all.
Hebrews 11:7
By faith Noah, being warned by God concerning events as yet unseen, in reverent fear constructed an ark for the saving of his household. By this he condemned the world and became an heir of the righteousness that comes by faith.
1 Peter 3:20
Because they formerly did not obey, when God's patience waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water.