I want to know if I can have a web application that serves multiple clients, for example, I have a web application which I changed the host name for to: http://MyIntranet.com, so can I have for example 2 clients connecting to this web application but each one of them having his own site collection with a different url? For example http://Toyota.com and http://Nissan.com but they are in the same web application? Will I need to play with IP addresses for the web app? Will I need to use HNSC?
Yes, you can have http://Toyota.com and http://Nissan.com in the same web application. You need to apply HNSC (Host-named site collection). The HNSC site collections can be created using PowerShell on SP2013.
You don't need additional IP addresses if all your site are using http only. All the site collections using different host name can share one IP address. However, you will need additional IP addresses if https is required. (well... there are some tricks to make multiple https site share one IP. But it is IIS configuration not SharePoint)