1) How hard it is to get into a class depends on the instructor and day/time. You'll be able to see the schedule about two weeks in advance but reservations open up 1 week prior at 12PM EST/9AM PST. If you're visiting from out of town I would set an alarm to book exactly at that time and make sure to have an account set up, class pass purchased, etc.
2) The most visible bikes are 6,7, 5 and 4. 6 is by far the most visible and is the one you see reflected in the mirror to the right of the instructor when watching a class. If you can't get those and want to be visible I would try 18 or 19.
It's a fun experience to visit the studio and I would recommend trying to squeeze it in if you're visiting NYC. As long as the instructors are not doing back to back classes, they will usually pose for a pic a few minutes after the class. Here's a
blog post with a ton of good info about the overall experience.
Here's a pic from one of my visits to the studio!