First off, thanks for checking out my game! I'm thrilled to hear you're into it. I'll do my best to answer your questions and hope you're satisfied with them.
How long does it take to travel?
The honest answer is "an indeterminate amount of time that the GM determines is appropriate for the narrative," but my rough rule of thumb is that the boat can travel out, do a round of fishing, and travel back in a single day.
How many times can you fish at any given hex/point when you finish current travel?
A similar honest answer as above, but my rule of thumb has been every player can fish once. The idea is that the result isn't a single fish but a type of fish abstracted across the day's catch.
Also what should a player roll when sailing?
They roll Traits like with any other Risk. I've always had my players designate a Captain to make the roll, whoever has the best Traits for sailing a boat.
How does the Risk work for fishing?
Yeah, the Fishing risk is the one that works a little differently. Instead of having a fixed TN, fishing is rolled against the chart. The better the roll, the higher the TN they clear, and the better the fish they catch. (Don't forget to have the GM roll 1d10 to check if their catch is Aberrant!)
I see TNs on fish that are cosmicly high, like TN100. What should players roll to try to get that number?
Things to consider: There are three standard traits they can roll (Profession, Personality, and Drive), each of which can give up to three dice, so your max possible roll on those dice is 54.
After that, you can look at Tools, which can give another three dice, increasing that max total to 72.
If they upgrade their boat's Tackle score three times, they get a flat +15 bonus for all fishing rolls, bringing us up to 87.
Now, what closes the gap is Dark Traits. The game is designed with the intention that the characters WILL take on Dark Traits. Since you can add as many Dark Traits to a roll as you can justify, I think it's pretty easy to argue adding Hysterical Strength (physically managing the line), Intuition (knowing how to esoteric patterns in fighting the best spot to fish), and Vision (watching the waters for any tell-tale signs of fish). Now, each of those at three points gives you another 54 points towards our max running total, bringing us to 141. And if clever players can argue for Violence, Insinuate, or Domination... good for them!
Then there is Egregore. If you have a group of four players with an Egregore of three, they can all give it to each other, adding nine more dice to everyone's pool or ANOTHER 54 points (because it's three players contributing to the fourth player), bringing that max to 195. Obviously, if there are more players, there can be even more Egregore... or less with less.
So, is TN100 ridiculously high? Yes. Do you need to get lucky to hit it? Yes. Do the stars need to align for you? Yes. Is it impossible...? No, it's just VERY difficult.
Also how many players can fish at the same time?
Everyone makes their own Risk.
How much time does it roughly take?
My rule of thumb is every set of rolls (every player rolling once) takes a 1/3rd of a day, leaving 1/3rd to travel out and 1/3rd to travel back if needed.
If you reach the given TN while fishing do you only get a fish that is behind that specific TN?
Yes, but the intention is that it's not a single fish but an abstracted "catch."
And again - while fishing should you really use your traits to add dice to your roll?
Yes, you use your Traits.
Can all players roll for fishing?
Yes.
What if you get multiple mythos events?
This is something that's not clear in the text and something I should revise (I also have an adventure I wrote I need to edit and publish... but my autism has shifted to a new favorite interest, so I need to wait for it to circle back to this idea.)
The GM checks for a Mythos event when the boat first enters the Waters. So if the players sail out to the Sunlit Ocean, upon arrival, the GM rolls 1d10, and on 9-10, they experience one of those events INSTEAD of being able to go fishing.