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>What is the maximum keyword numbers can i put in a txt file ?
Just limited by your memory, no limits here.
Whats setting do you mean? Each system/network is different so I can not really give you number of what to put in "thread limit" e.g.
>I am targeting of scraping millions of emails.Do i have to use proxies just to scrape emails as i have huge keyword list?
The keywords are used legally I hope? If you just use random keywords here they are of course not really specific to the topic. If you just want any email than yes, I would suggest you to get proxies. Because search engines might ban you sooner or later if you use them too often.