Is there any limit on number of devices you can install Apps on from Amazon App Store? I recently started using the Amazon App store, and noticed that it copies the apps I already purchased or downloaded to other Android devices if I sign in using the same account. Is there any specified limit on the number of devices that this is supported on? If I have 10 Android devices can I buy an app once and install it on all of them?
Yes, you can install all the apps you buy from the Amazon appstore on any device which is linked to that Amazon account.  It operates just like paid apps in the Android Market which are available to all the devices associated with that particular Google Account.  

I have 3 Android devices and all they are all linked to the same Amazon account and the same Google account and when I purchase an app through the Market or the Appstore I have it on all three devices.