If you really meant to say ISP account, I am not sure what you meant.
The number of domains you can host on a hosting account or IP is limited only by the restrictions created by your hosting company. If your plan allows for addon domains you are good to grow.

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If you are on a dedicated server and have cPanel or WHM you are also ready to go.
The process is slightly different on the set up, both both work exactly the same as far as the web visitor and SE are concerned. The two or twenty sites are different entities.