All AWS accounts have a "limits" which have the default limits as to how many instances that you could launch in that region.
The reason is so if you fuck up a scaling script for example you can't launch 1000 machines and take all the capacity and then bitch that you won't pay for it.
It's a stop gap.
However, aside from the hard limit of 100 S3 buckets, all other limits are configurable at the request of your AWS rep
Hmm... I didn't know this. The last time I asked they said it would never happen. I was told the original reason was that all buckets had to have a unique name
The reason is so if you fuck up a scaling script for example you can't launch 1000 machines and take all the capacity and then bitch that you won't pay for it.
It's a stop gap.
However, aside from the hard limit of 100 S3 buckets, all other limits are configurable at the request of your AWS rep