You are stamped in for 180 days at a time, there is no maximum amount of stay per year. You can exit and re-enter Thailand immediately for another 180 days, or apply for an extension at immigration for another 180 days.
Nobody knows what will happen come tax time. It will certainly be a registration shltshow when/if it does happen. I personally am in no hurry to get a tax ID number until instructed to do so. My 2 cents.