I just want to connect it the first time to upload a sketch and test it.(its going to be put in circuits for lights home applications)
For something like this, you would actually want to place the individual ATMega controllers on a serial bus of some sort, or communicate with them wirelessly, not run a USB->serial converter to each one. By putting them on a bus (say RS-485 - but there are other options), you can have a single one of them hooked up via the convertor cable (the "master") to which you send the commands, and then it would relay the commands to the rest of the system nodes (the slaves) on the network.
With that said, what you are trying to do is not going to be inexpensive, unless you are willing to integrate components (some SMD based - and even doing this may not get you in the "under 10 euro" range) into the standalone as well. For instance, for RS-485 connected or wireless nodes, you would need to embed not just the microcontroller, but also the communications hardware involved - whether it is an RS-485 bus transceiver IC, or a wireless RF transceiver board).