The number of IPs used by Crawlera varies constantly from thousands to tens of thousands. This number, however, is not very relevant, as long as it stays above a certain threshold, which guarantee the required volumes to crawl websites.
This question often arises with proxy providers because the number of IPs determines how fast you can crawl a website. With Crawlera, the number of IPs doesn't matter because Crawlera (not the user) will throttle down requests so there is a global limit upon which adding more IPs won't increase the crawl speeds.
These limits are carefully tuned and always revised by Crawlera engineers (and automated algorithm) to make sure you can crawl as fast as possible without causing any disruption to the websites.