Obviously you can manually limit how many folks can buy-in. You can easily hard-code that into the layout, too, to show your players. For example, in the Tournament Info cell from the default layout:
<buyin> Buy-in, <rebuyinfo>, <addoninfo>
You could change it to read:
<buyin> Buy-in until round 6, <rebuyinfo>, <addoninfo>
You could also do something like this to show how many rounds are left to buy-in:
<buyin> Buy-in (<expression text="max(7 - roundNum, 0)"> rounds left!), <rebuyinfo>, <addoninfo>
It's not dynamic, so you'll have to change this if you change the level at which buy-ins end.