I bought a new VPS using WHM optimised and it's just one domain, one IP.
Are there benefits of many domains sharing the same IP (remembering the IP is for the root) or are there any disadvantages to it?
Is it preferable to create and assign a new IP to each new domain?
There are a limited number of IPv4 IPs so you should share where possible.
Services like SSL require 1 IP address per seperate certificate. Wildcard certificates may not be suitable and are more expensive. So in general this means 1 IP to each domain using SSL.
If you dont use SSL for your domain, it is ok if you share 1 IP address for all of your domains. There is no problem. But, if you need SSL on one domain, you need at least 1 static IP.
Taking into consideration the costs of IP addresses due to shortages, its much more economical and easier to manage if you just share the same IP address for all domains.
Web server control panels like cPanel and Plesk handle all the virtual host issues for you when you create an account, and generally the default account template setting is to share an IP address.
As others mentioned, domains only need a separate IP address if an SSL certificate (for secure connections, shopping carts, etc...) is purchased. Also some database and server applications require a separate IP address. But if you don't have these needs, I would feel confident in using a single IP address for your server.