What is the rack of chips centered on the top of the table? I noticed the number changes frequently
You use it to add more chips to your game from your bankroll. You can reload your stack as you play.
The numbers change because the different tables have different buy-ins and have a minimum buy-in amount so the amount of chips you can bring to each table differs.
There is a maximum amount of chips you can bring to a table as well. Just enter the amount you want to bring to the table up to the maximum allowed, which will be displayed for you when you click the rack of chips at the top of the table.