>>713430673So there are some hard limits - currently you can't have more than one set of tracks and just wheeled designs aren't possible (at least in vanilla -both are on the dev roadmap and I think some people have made mods for both, but it's very much in progress).
Multiple guns is fine, pic related carries a 90mm and a 76mm secondary (it also weighs nearly 99 tons due to the amount of armor on it) The latest update added the ability to 'link' guns so they're both controlled by the same crewman.
There's no round variety yet, just standard APHE so far. You can modify the caliber and shell length (aka the amount of powder). There's also not really any different armor types, it's all one kind.
Like I said, there's weight limits for the scenarios plus a number of 'eras' (WW1, interwar, early war, midwar and late war) that determine things like available engine power, suspension, cannon power etc.
Your best bet to see what's possible is to look on the >reddit or on youtube.
Disclaimer that the game is still in early access and EARLY, like 0.2 and it's a sole dev I think.