Twenty at most it is the easiest way to remember keeping it a good amount but not to high
'Exert' means to apply force on influence over something else, this can be in a negative or positive way. In this quotation, it means that language, if used correctly, has the ability to express a 'hidden power'. It's also worth mentioning that the simile is relevant as the moon has an influence on the level of tides on Earth.
I say it would be the second one because to show possession you have to have an apostrophe.
