AWS IOPS limit during load test

Question

I have been carryout load test on an API service deployed AWS EC2 instance. The service is deployed on two boxes (each c4.8xlarge) and is being load balanced. Test is carried out using JMeter in distributed mode where in test agents are also aws and are m4.4xlarge machines. There is no EBS storage used.

Each sample response is of size 315 bytes and I receive about 12k/sec throughput with testing run on 200 threads. In anticipation of receiving higher throughput I increased the threads to 450 but this did not show any difference in test results i.e. either throughput or response time.

What would be IOPS limit when I am not using EBS? Also,

I measured load avg, cpu utilization using top command and the count is way low on each test run to be considered a blockers.
There was no EBS used on EC2 c4.8xlarge instance
There were about 300 write operations of .1kb each and 24million read operation of 315byte each

you don't say how big the EBS volumes are which affects the IOPS see docs.aws.amazon.com/AWSEC2/latest/UserGuide/… You don't say if you've looked at top or netstat or cloudwatch or other simple tools to see if there are any other resource limits being hit — Vorsprung
+1 for Vorsprung's comment, we need disk size to answer this. Also just to add: if you are only trying to get high disk iops, you can consider to use RAID 0 striped disks (the next bottleneck would then be the bandwith, throughput, etc) — Tom