looking at this line https://github.com/mattmoor/branch-api-plugin/blob/f4890e44a3686e5ae19d0a09ee408148a8f1fb16/src/main/java/jenkins/branch/MultiBranchProject.java#L2293

this is the entire code snippet from the git repo - notice the hard coded number in the second if statement....

public CauseOfBlockage canRun(Queue.Item item) {
            if (item.task instanceof MultiBranchProject) {
                if (indexingCount() > 5) {
                    // TODO make the limit configurable
                    return CauseOfBlockage.fromMessage(Messages._MultiBranchProject_MaxConcurrentIndexing());
                }
            }
            return null;
        } 

I see the maximum indexing capacity is hard coded.

looking at the next line shows a TODO line with "make the limit configurable".

in our environment, we have hundreds of multibranch pipelines and the configuration of this parameter is critical to increase the amount of scanned branches/tag in parallel.