I would like to know, how low is safe so that search bots and normal visitors do not get blocked?
As low as you can get without search bots and normal visitors being blocked. (i.e. "There's no way to tell without empirical data - It depends on your site, the kind of browsers/crawlers being used and how many simultaneous connections they'll attempt to open, whether or not users are behind proxies/NATs that make many users appear to come from one IP address, etc. etc. etc.")
Practical advice: If you want to use an auto-ban like this err on the side of false negatives (allowing attacks to continue) rather than false-positives (banning legit users). 200 simultaneous connections from one IP seems to be a reasonable value, provided you don't have hundreds of users behind a proxy somewhere all looking like one IP and all hitting the site at once.