According to AWS' FAQ:
http://aws.amazon.com/ec2/faqs/#How_many_instances_can_I_run_in_Amazon_EC2
The limit of Spot instances is now 5 per region, which sounds incredibly low.
Quotes from an older version of the FAQ and answers on StackOverflow/Quora says AWS did limit to 100 Spot instances until recently.
" You are limited to running 20 On-Demand or Reserved Instances, and running 100 Spot Instances per region. [...]"