The typical answer to this is "build the type of game you want to play." This is completely and entirely true—but unhelpful to a beginner. Here are a few starting suggestions. Try some things out and adjust from there to your liking!
- The game plays well with a wide range.
- A good starting point is enough cover that most frames can be deployed into cover, plus a few extra bits.
- It's generally accepted that "light" cover takes only one hit. "medium" cover takes 2–3 hits. Anything soaking 4 or more would be "heavy" cover. It's a good idea to have a mix of all types. A game would be fine with all light and medium cover, but you should probably avoid having a great deal of heavy cover all over the board. Heavy cover slows the game down.
- It's good to also mix up the space between cover. Avoid the urge to scatter it evenly around the field.
Here are some good examples.
Small bits of scattered cover work fine.
viewtopic.php?f=17&t=6832viewtopic.php?f=17&t=4278Small-to-medium scattered cover is fairly popular, too:
viewtopic.php?f=17&t=6254viewtopic.php?f=17&t=6071This one has a lot of variety in weight and density:
viewtopic.php?f=17&t=6247Great big set pieces are impressive and lots of fun, but sometimes you have to enact special rules about how to destroy them so they don't become impenetrable:
viewtopic.php?f=17&t=5316viewtopic.php?f=17&t=5337viewtopic.php?f=17&t=6898Hope that helps!