If you're looking to experiment -- I think the best answer is talk to AWS support as this isn't a normal case that is allowed by AWS accounts.
Check out
http://aws.amazon.com/ec2/faqs/#How_many_instances_can_I_run....
To quote it:
"
You are limited to running 20 On-Demand or Reserved Instances, and running 100 Spot Instances per region. Cluster Compute Eight Extra Large instances are limited to running 8 On-Demand instances per region and Cluster GPU Quadruple Extra Large instances are limited to running 2 On-Demand instances per region (Currently cc2.8xlarge and cg1.8xlarge instances are only available in US East (N. Virginia))
"
However, these are the default limits -- AWS will up your account limits by quite a bit if you just ask. This said, max I've ever seen AWS approve was 400 on-demand instances per one region (I think this means 2800 instances total over 7 regions... never saw if the limit applied to all regions). Also, likelihood of having a 10000 spot instance request fulfilled is probably near zero if I were to guess even if they let you make a request that big without first clearing it with them.
I take it you're also familiar with the cost of running that? The absolute minimum you could possibly be charged using small instances is $1,200 -- partial hours are rounded up.