Actually, thats really good question! I had people ask me that before, but in all honesty, its hard to say for sure and here is why. I agree 100% with @Chris_Atkinson ...it all depends on traffic and how much specific processes will consume. I worked with client who was using all those blades and worked fine with 16 GB of ram, but personally, I would suggest 32 GB. Not telling you that cause I want you to spend more money ( I do NOT), its simply because its better be safe than sorry as they say. Yes, you can always add more RAM later, but I like to be proactive rather then reactive. Its always more prudent to have more RAM and not use it and than not have enough in the first place.
Also, keep in mind, threat prevention blades and related processes tend to cosume more RAM than regula ones.
Hope that helps!