public BlockingTaskExecutorBuilder numThreads(int numThreads) {
        checkArgument(numThreads >= 0, "numThreads: %s (expected: >= 0)", numThreads);
        this.numThreads = numThreads;
        return this;
    }
Sets the number of blocking task executor threads.
core/src/main/java/com/linecorp/armeria/common/util/BlockingTaskExecutorBuilder.java