The size range and number of SLCDs carried by a climber will vary on the rock type they typically climb and how flushed they are with cash. To give you a guide of what typical racks will comprise of here are a few examples
FULL COVERAGE SLCD RACK
This is the typical cam rack a climber would have at their disposal, taking all or part of it on a climb depending on the rock type. This kind of rack would probably be built up over several years, and often the overall rack is composed of shared units. The rack would go from finger tip (.5 inches) to fist (4 inches) would probably be spanned by 9 units. In the past the typical UK rack would be Wild Country Technical friends 0 to 4, but these days there is much more variety, and so it may comprise of two Wild Country Zero’s (size 4 and 5), 4 Camalots (.75 to 3) and maybe a size 5 technical Friend (you could add a second Camalot 2 to increase the racks coverage). A lighter option would be to go for a mixed rack of DMM SLCD, maybe getting 4 3CUs (.5 inches to 1.5 inches) and 5 4CUs (2 inches to 4 inches) creating a very light rack ideal for mountain routes.
BARE BONES RACK
If you’re skint or are slowly building your rack then what should you buy? Well SLCDs usefulness starts in the middle sizes (2 inches) and get less important as you go outwards, meaning the dwarf micro cam and the cam as big as your head should be the last units you buy. As a rule of thumb you want to end up with three units that cover your main bases, namely a 1inch, 2inch and 3 inch piece (equivent to WC 1,2,3). And once these are bought you fill in the gaps, adding a 1.5inch, 2.5 inch, and then .5inch and 3.5 inch pieces to the rack. This approach is also worth remembering when you’re trying to build up a lightweight rack, perhaps for a alpine rock route, although on granite I often go for 1.5, 2.5, and 3.5 inche pieces as the larger unit often comes in handy for wider cracks (your nuts can handle the smaller stuff).
If possible stick to one brand for the bulk of your SLCDs, as this allows you to build up an instinctive feel for what size will work best in a given placement. When I say the bulk I’m talking from around ‘off finger’ size (1inch) to fist (4inches), which makes up the most used cam sizes, meaning cams that need to be robust, reliable and easy to handle. Cams above and below this size can be of the same design, but personally I find that looking for more specialist designs pays when the placements get tricky. My favourite finger, tip and smaller sizes are narrow more flexible stemmed cams like the CCH Alien of Wild Country Zero, which go in far more placements then say an equivalent BD Camalot of WC Friend. Why wouldn’t you pick this style of SLCD for all sizes? Well these cams tend to be more complex and finely tuned, making them more expensive and less robust over years of abuse. In the bigger size you may want to get a cam with more range, or lower weight then your standard sizes, for example I would often choice a Friend 5 over a Camalot 4, as it was lighter and cover a larger size range for some climbs.
Once you’ve built up your main force of SLCDs don’t be afraid to add extra units to the range if you find one or two sizes are getting used up more then others. common sized SLCD that get used more then others range from 2inches to 2.5inches, meaning you may want to double up on those sizes. If doubling up on one or two sizes I’d look for another brand that offers a slightly different range, so that you can overlap with you main set. A slight detail worth noting when mixing cams is that that they still rack nicely, ideally sitting in the same position as each other. If they don’t, say with one cam hanging down 3 inches lower then the others, there can be a tendency for the cams to get tangled up.
If you’re heading up some big walls then you’ll probably be needing a double set of cams, and the best option is to simply double up on your existing rack (that way you’re already dialled in to all the sizes, colours etc).