We know that we have below limit for reserved instances for EC2
There is a limit to the number of Reserved Instances that you can purchase per month. For each Region you can purchase 20 regional Reserved Instances per month plus an additional 20 zonal Reserved Instances per month for each Availability Zone.
For example, in a Region with three Availability Zones, the limit is 80 Reserved Instances per month: 20 regional Reserved Instances for the Region plus 20 zonal Reserved Instances for each of the three Availability Zones (20x3=60).
does this same limit also apply to RDS or do we have separate limit for RDS ?
For RDS, up to 40 Reserved instances can be reserved. Beyond that, you are required to fill the form.
Official documentation