Use settings of Scrapy Spider as provided in command line arguments or sent by ScrapyD.

scrapy.py

import sys

class MySpider(scrapy.Spider):
	name = 'my_spider'

	CONCURRENT_REQUESTS = 10

	for x in sys.argv:
		if 'CONCURRENT_REQUESTS=' in x:
			CONCURRENT_REQUESTS = x.split('CONCURRENT_REQUESTS=')[1]
			break
			
	custom_settings = {
		'CONCURRENT_REQUESTS': CONCURRENT_REQUESTS
	}