Hi, I'm very new in this forum and I have one very simple question. How many Arduino boards I can connect to a single PC (USB) and interface with them with only one flash movie? I already start my project with Arduino and Flash, but I find out that number of I/O of a single boards isn't enough. I need at least 5 Arduino boards. My guess is that number is limited only to number of one-digit serial ports availability – am I right? Thank you.
If the issue is that you don't have enough I/O pins you could use a chip called an "I/O extender" to get some more.
If you tell us more about your plans we can be more precise in our suggestions. Info on your operating system is probably also relevant to your question.
--Phil.
If your computer has 8 or 10 USB ports you can connect 8 - 10 arduinos to your PC, however that will be complex and expensive
A better idea might be to look at the sanguino, Illuminato or other boards with more IO pins 
You can get the sanguino from http://store.rrrf.org/index.php?main_page=product_info&products_id=81 or http://www.wulfden.org/TheShoppe/freeduino/sanguino.shtml
The Illuminato is available from liquidware but is currently back ordered http://www.liquidware.com/shop/show/ILL/Illuminato
The sanguino has 32 IO pins and the Illuminato has 42 IO pins