Sorry Alejandro,
I didn't state it clearly.
API Key on portal is exactly the ID of "Application", an "Application" can have one or few APIs, their ID is called API ID on portal, and it has one to one relationship with a policy ID on gateway.
As the rate limit depends on the API plan, or "policy ID + API key", or API ID, it has nothing to do with API key (the ID of Application). That's what I meant "API key should not be a concern".
And yes, if the rate limits set as 5 req/sec as per the API plan, 10 clients cannot access the same api at the same time. But this is working as expected as per the API plan. If you think it's not enough, you should assign proper API plan to the API by editing the API on portal and modify the "Applicable Plan(s) *" field. The API plan can be preset by admin under Home -> Dashboard -> APIs -> API Plans
Regards,
Mark