When I render the clouds with the DPI set to 150, I look at them both from afar with the entire image seen on my laptop screen, and up close where it is about as true life accurate in terms of size. No matter what setting, it looks like crap. I did a Google Search to see if something could be done about it, but there are no options for the Render Clouds tool. Apparently, Photoshop just takes the DPI you are using and fudges around some formulas to determine the number of clouds to render in the space you chose. So the only way to reduce the number of the clouds is to reduce the DPI when they are being generated, then increase the DPI back to what your final product will be.
I'm using Photoshop CS5 as that's a version I've had for a while (Long story, but somebody I used to work with had gotten a copy of it for free, but had no use for it so he just dumped it on to me. For various reasons, I cannot upgrade this version).