Say you are running 10 good websites
 

Would you put it under one Powerful dedicated server or split it into 2-3 vps or even 10 shared accouts with 10 different hosts? tnx

It depends on the type of sites.

Paysites only: dedicated server

Anything else: 2-3 VPS

I would be sure to keep WordPress blogs in a separate VPS to ensure the safety of anything else as they tend to be the main security risks.

depends what are the sites?

Define 10 good websites?
How long is a piece of string?


Take it from me, or not if you like.
SEO wise it doesn't matter.
There are a lot of old wives tales doing rounds considering SEO about this.
And I can prove different with almost anything 'people belief'.

I would say it depends on where you stand and are with your sites.
See it this way:
You start with one site on a shared host.
You build another one...
You build another one...
You build another one...
Then comes the moment that a VPS is more cost effective, suiting and technically easier.
You can upscale that.....
Till you need dedicated or dedicated cloud.
Once on dedicated you need either load balancing or CDN solutions....

How long is a piece of string?
Long enough to tie what you need to be tied!

Personally, so long as it was affordable, I would try to split them up across a few hosts. This way if one host goes down, all of your sites don't go down. If you have trouble with a host of any kind, it doesn't effect all of your sites.

Kind of a not putting all your eggs in one basket thing.

Yeah, I need to start listening to good advice as the aforementioned. All my sites are down due to a MYSQL error and the disk being full on the VPS. So, little to no income since yesterday and other people are complaining about their sites being down. I'm now having to spend yet more money.

if you ever wanted to sell links on your sites, having them on different hosts with different ISPs might be advantageous

And yet you promote DO in your sig.

How would you have felt when you where semi-managed and just sent an email...
Finding quite soon the issue was solved and how to prevent that in the future?

I run wordpress sites. I don't sell links or anything. I don't trade links either even amongst my sites.

To me, The benefits of running 1-2 sites per dedicated server
Pros
1. Good for SEO (google loves speed)
2. Plenty of room to grow traffic without downtime.
3. If one sites receives "PENALTY" from google, it doesn't spread to your other domains. (yes, "PENALTY" even though I did no link buying/link trading. You can never be safe from google)
4. Easy to setup new accounts

Cons
1. Expensive
2. If one site goes down, so do the others
3. Theres a chance if google penalizes one of your sites, that others will be affected.
4. Once you start getting 10k hits using wordpress, your shared hosting account gets suspended thus resulting in downtime.

Thats how I see it with dedicated servers vs non dedicated

If you do interlink them, it does not matter as you say because Google easily figures out those 10 sites belong to the same entity whether they are hosted on 1 server or on 10 different boxes.

If you use similar inbound links and don't interlink your 10 sites, it may matter a lot depending on some other factors - mainly whois info, hostmaster emails and linking codes and other foot prints.