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