Having been asked about Terrain/Cover on the Discord channel, I figured I'd better write a blog post about it. I've done posts about this in the past. But let's take a slightly different view this time.
1. The Look.
We can take cues from real world architecture here. Different neighborhoods within cities have different looks depending on the architecture styles and building materials in use at the time of their creation. Old neighborhoods are brick, not so old neighborhoods are cement, newer neighborhoods are glass and steel. Building materials define color. Style defines the look. So work forward from there. In my Sci-Fi Outpost setting I worked with primarily white and lime pieces to make it look like everything was made from similar materials. If you have a big enough table you might even have enough room for different neighborhoods on that table. Outside cityscapes things are a little different. In the real world vegetation is primarily green due to clorophyll. If you want something a little more sci-fi, we can change colors away from green. I'm working on an idea where plants are made of white and gold pieces. Vegetation in the Oasis setting is either blue & purple or red & black. Do what you want, just have an internal consistency to what you do. Also, don't forget that a good battlemat or piece of felt can add to the look of a table as well.
I'm going to make up a formula here. 60% of your Cover should be 1 or 2 Hit pieces, 30% of your Cover should be 3 or 4 Hit pieces, and the remaining 10% should be 5 or more Hit cover. Keep in mind that really heavily built Cover can be a problem. The Milano in this game looked really good but barely looked scratched after taking damage that would have crushed most targets. I don't have a formula yet for how many pieces of certain sizes of Cover you should have. There should be a fair amount of Cover that can only hide one Frame. There should be some that can hide most of a squad and, a few that can hide an entire squad. Maybe call it 50% small, 30% medium, and 20% large pieces. Of course all these numbers change if you want to have a super frangible battlefield or an almost indestructible one.
Looking at my Actual Play pictures on Flickr, I count between 14 and 26 pieces of Cover on the table at any one time. Depending on the size of the table, this can be either a roomy or cramped battlefield. This is why it can be hard to set down a number for what you should use. Total area of the cover in relation to the total area of the table is a big variable.
Generally the arrangement of Cover on the table is up to all the players before the game starts so this note is more of a "How Not To" than anything else. In our Missile Base Omega game I put a fence across the table and tall structures behind that fence with shorter cover on the other side. The Defender set up in the structures pushing the Attackers to the other side of the fence. There was one incursion through the fence towards the Defender and the rest of the combat occurred in the mostly open area. I feel the fence discouraged people from trying to go over or through it -- despite being easy to destroy -- and this led to the fighting being bottled up on one side of the table.
One more caveat, all of this depends on what you're trying to accomplish and the supplies you have available. Aiming to have an awesome looking table is one thing, feeling guilty because all you can do is put some simple walls on a bare table is another. We're all here to have fun. Nobody is going to judge. I hope this helps people and I'm going to try and do more posts in this vein.