Depends on the kind of sps.
-Plating montis usually have a lot of room, but are near the bottom so they don't shade.
-Encrusting montis are either on their own rock, or have plenty of room to grow in before hitting other frags
-Stag acros/torts/verticle growers usually a couple inches around since they grow up primarily as opposed to out.
-Encrusters usually have 3 or more inches around them (for example pink lemonade or red planet usually encrust quite a bit before growing up) so they have room to encrust and then grow up
-Tables are usually on their own spot off of a verticle face or steep sloping rock so they can grow off in their table form without shading other acros too much.