Hi Laurent,

given that any OpenStack installation has its peculiarities, you could find detailed info on specific architectures here.

I will try to answer to your questions:

1. You could manage 20 compute nodes with a unique controller. If you want HA you need at least 3 controllers. There are many ways to implement this architecture; for example with Canonical, as you install controller services as containers, you could decide to mix controller and compute functions. With Mirantis (that uses Fuel as installation tool) or Red Hat (that uses TripleO) you have separation of roles.
2. Yes, it is possible
3. You need at least 1 interface but best practice says to segregate traffic. Advanced setups could require 8 interfaces. You could find additional info on official documentation.

4. Every installer has its own pros and cons:

    - Canonical Juju requires the installation of Autopilot and MAAS for machines management; it is simple, and you could choose to deploy OpenStack services with standard approach (VMs) or containers (LXC); 
   
    - Red Hat TripleO is very straightforward and well documented;
   
    - Mirantis with Fuel; also in this case the approach is very simple and the tool very reliable.
   

There are for sure many other installers. This list is not intended to be a complete list!

1. I advice you to read official OpenStack documentation, especially Arch Design Guide, Networking Guide and HA Guide.

Find more info here: Canonical, Red Hat, Mirantis

If you want there is also the possibility to install it manually; even if it is the best way to fully understand modules interactions and specific configurations I would not say it is the best choice.

BR, Antonio