You can sync as many devices with the Informant application to your Informant Sync account as you like, there is no limit.

One thing however to keep in mind that when syncing multiple devices to the account you will want to consider using a longer autosync interval.  For example if you have Informant on all devices set to autosync every 5 minutes the chances of having sync operations collide and potentially damage your data is greater.  Where a 15-30 minute autosync interval will reduce the opportunities for a collision to occur.