Same server or same account?
Because you can have a reseller account and host them on the same server but on different accounts. That would be better.
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Assuming you're using cPanel based hosting, you can host multiple domains on a single account using the addon domains feature. Of course this will depend on the number of addon domains allowed in your hosting plan.
If you manage several domains on one account it can be easier from an admin point of view as you can control all of the various sites from one central location.
This is do-able through c-panel.
You should not have any increase in SPAM/DDOS probability hosting on one cPanel account when compared with hosting each domain separately on the same server.
It may not be a bad idea to consider external DDoS protection for your entire server or ask your hosting company if they offer such protection. People above are right, when using shared infrastructure, if one domain gets attacked, your whole server may/will suffer.
Advantage of having them all in one place: One management location for everything.
Advantage of having them separated (say, in a reseller plan): If one of them is hacked, the hacker doesn't get automatic access to the files for the other sites; if the host limits resources (say, amounts of CPU and RAM) per account, each domain would get its own resources to use.
I use Digital Ocean. VPS hosting starting at like $5/mo. They have an initial limit of 10 (mine was 15 for some reason) and you can easily spin up new droplets and they have a couple different data centers to choose from.
They have good tutorials but it's somewhat expected that you sort of know what you're doing from an administration standpoint if you went with them, but I like them a lot. They have various distros to choose from and they do have some images like a server with wordpress already installed, etc depending on what kinds of sites you were hosting.
That being said, you can have multiple private servers in multiple data centers for $20/mo or less. They're probably not for those that are afraid of doing things themselves but for those that are fine with that, terrific option.
This is, in my opinion, a personal decision. It's easy to make the argument that if you've selected the right provider, and have done proper due diligence, then you should be confident that hosting your entire infrastructure is okay.
It all depends on your priorities actually. If budget is an issue then you could probably go for a shared/reseller plan and host all of them on that one server. If security is your main concern then you should probably host the domains on different servers to prevent total failure during downtime, hacking, dos attacks etc on all your websites together. Also you could check with the hosting provider as to what are their security measures to protect your website if you host with them.
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Depend what is the resource utilization of each domain. if you dont have any much traffic on any of those domains its better to each all on one. Would recommend to host heavy load domain with individual accounts.