Depending on the number of users and the amount of traffic you would expect for Portal itself, it may not be advisable to put Portal on any of the machine(s) within your ArcGIS for Server site. Your AGS services are going to be responding to REST requests, thus consuming system resources alone. Adding additional load (i.e., essentially ArcSOC processes) to the same machine as Portal to process service requests could be crippling to your server. Evaluate the number/type/complexity of the GIS services you publish in conjunction with the expected load that will hit AGS and Portal; then make a decision with the available system resources in mind (i.e., CPU, RAM... and also disk space). You may find that Portal should go on its own machine based on your findings. What you need to avoid is pushing your server to capacity if both applications are fighting for resources.
On another note, I cannot remember how Portal is licensed by Esri. ArcGIS for Server is licensed per CPU core, but Portal may have a different licensing model. You should verify this with Esri before deciding to involve a second server in case you double your licensing costs (unless you have an ELA).
