With our Participant Count price increases always has the downside of people being upset they did not get the price they wanted. The default way it works is the first X people who click Submit on the Checkout page to process their transaction. The problem with this is that we redirect the participant back to that checkout page to tell them they need to re-enter their credit card (for PCI security reasons) at the new price level.
We have added an option (on Miscellaneous Settings) to set a window for people to complete the checkout page before the price expires. This means that more than your limit would get the special pricing – so be careful on this.