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I would say around 24 or 30 is a good number, of course the more the better. Personally I'm not a fan of Speedrun achievements, you know the ones that require you to beat the game in X amount of time, and I've come across many other steam users who shared my dislike of Speedrun achievements. I've always enjoyed taking my time to fully explore a game and not rushing through it, although I know there are some people who do enjoy speedrunning. Either way the addition of Steam Achievements will give you access to a lot of steam users who will only buy games that have Steam Achievements, which is quite common, and considering how awesome your game looks the addition of achievements will only enhance the experience and encourage steam users to buy the game...thanks in advance for you time, keep up the great work and good luck ;-)