If you have checked out this blog post: SP Dev Machine, you will notice that the article refers to a single development machine for all SharePoint 2013 Development. You can add a 2nd VM to the mix for Office Web Apps, but that is largely just an option for everyday development.
However, if you refer to various other consultants or professionals, most will recommend 2 VMs at a minimum for SharePoint 2013 Development.
Why is this?
The main reason is to separate the functionality of the domain controller from the actual SharePoint 2013 Development environment.
In this manner, you can easily build out larger SharePoint Farms all using the some domain controller VM without having to worry about messing up your original SharePoint 2013 installation.
The biggest reason that I find for separating out this functionality is because of the extremely heavy emphasis many organizations are placing on Claims-based authentication. Numerous corporations are now adopting ADFS (Active Directory Federation Services) to handle SSO for many of their applications, so it would be logical for a Claims-based SharePoint developer to eventually have to set up his/her own ADFS Server. In this case, the most logical choice for setting up an ADFS Server would then be the existing and available domain controller.
The 2 VM segregation also provides you with a little bit more leeway while testing Cumulative Update patches on your SharePoint 2013 environment. Of course, before you apply any Cumulative Update on a VM, you always want to have a snapshot before installing the patch, but separating your SharePoint 2013 environment from your domain controller/ADFS Server provides an additional safety net in case something goes wrong. Worst case scenario, you would have to completely re-build your SharePoint 2013 environment, but you would still have your domain controller/ADFS Server intact.
Therefore, if you have enough memory to support 2 SharePoint Development VMs (16 GB minimum but preferably 24 GB or greater), I would definitely recommend that as the way to go.