which only needs to be increased on a Domain Controller (DC) if you need to increase NTLM throughput for users from trusted domains, in other words this does not need to be increased if the customer only has a single domain.
BCAAA sends all NTLM requests to a DC from its local domain, regardless of user. If a request is for a user in a trusted domain, then the DC forwards it to a DC in the user's domain. That's when MaxConcurrentAPI on the DC comes into play.
As a result in a single domain environment there is no need to increase the MaxConcurrentAPI as there will be no outbound requests to trusted domains
MaxConcurrentAPI in a multi trust domain environment should be set on the Windows BCAAA server(s) and on every DC in the site or domain (this is not true for IWA direct where MaxConcurrentAPI only need to be increased on the primary and alternate DCs.
Bear in mind that it is the Windows server that BCAAA is installed on that chooses the DC it will talk to, BCAAA has no say in this process. Sometimes Windows will pick the fastest DC, but not necessarily the DC in the best position to handle the load of NTLM request. If Windows changes DCs for some reason (the DC goes off-line, the BCAAA server is rebooted, etc.) and the new DC is slower, then the customer may have an outage. In many cases, Windows won't switch away from a slow DC without admin intervention, because Windows doesn't know there is a problem ie the DC is still responding to requests, albeit it more slowly.
While it may be tempting to disregard MaxConcurrentAPI in a Kerberos environment, remember that Kerberos will fall back to NTLM if it needs to and LOCAL users will also use NTLM
As to what value to use, this will depend on a number of factors such as
the version of Windows
how much memory the servers have
the value of numthreads in the bcaaa.ini file
Blue Coat has test with numthreads as high as 128, which would correspond to a value of 64 for MaxConcurrentAPI
As a general rule of thumb the value of MaxConcurrentAPI should be half the value of numthreads in bcaaa.ini