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but if there is a problem , all websites will be affected.
That'll happen only if any of the following occur:
1. You use their DNS for all domain names and they experience trouble.
2. Your account/s lock/s up for whatever reason (e.g. login unsuccessful, billing problems, etc.)
3. Legal issues occur. (e.g. Court order for all domains registered in your name.)
Other than those (and whatever I might have missed), there's no reason your websites will be down even if you use the same registrar. Many people use the same registrar solely for domain management and their website/email/etc. hosting elsewhere.
Registrars don't necessarily deal with webites, and I'd certainly recommend that you register your domains with a registrar and host your websites with a good quality webhost, who is independent of your registrar.
OK this does mean having a separate webhost and registrar. So why do I recommend that? Well it's basically for insurance in case you have any kind of dispute with your webhost (it happens). They cannot lock you out of your registrar account and you can simply sign up with a new webhost and point your domains to the new webhost, with minimal downtime to your websites.
1) If you do any SEO stuff, having all your domains with one registrar can raise a red flag with the search engines, even if you use whois privacy (which I do for all my domains)
2) As far as risk, I believe it is less risky to have more than one registrar, because if the one you choose has problems it's either all your domains or some of your domains. Having multiple registrars is more work, but if you manage your info well it's actually not bad at all. This assumes, of course, you choose good companies to work with (a trial and error proposition)
Maybe you don't need that for 100 .coms but you'll find it is very useful if you grow your portfolio or start mixing in ccTLDs (where prices really change a lot with different registrars).