exactly how are you positioning your SB's? i mean are they forming a square one at every "corner" of the sun.........if a sun had corners that is. or is there two next two each other to guard one anothers flanks? also are they close to the sun or are they pushed out toward the edge of the gravity well so they can catch as many ships coming through the phase lane??
It depends on the star, its position to other stars, and the number of phase lanes connecting to the star.
If the star in the the center of the map (center of a cluster) and has multiple phase lanes to its solar system, then four SBs in a square should do the trick. In this case I max out the hanger space upgrades for all the SBs, so they can still provide support to one another, followed by 2 of armor and 2 or 3 of "insert choice here". 56 squadrons (80 for advent*) of strickcraft is enough to hammer most fleets.
If your star is out by itself, such on a 2 or 3 star map in which all the stars are in a line, then placing all four SBs along the 'inbound' side of the star isn't a bad idea. In this case I use the same set of upgrades as above.
And finally, if you are lucky and the star only has 1 or 2 phase lanes leading from it, then by all means focus your defences alongside them where fleets are likely to jump to or from. If you do this, your starbases might be close enough to provide support for each other. If they are, then you can diversify your upgrades a little bit more if you choose.
The biggest threat to you star-based starbases are the TEC's Orogovs and the Advent's "Starfish". You take your eyes off the battle for only a second, and a small fleet of these can move in and empty our your star's grav well for you.
* All 56/80 squadrons attacking the same target provides insti-death to any frigate or cruiser, and knocks out Caps with only 2 or 3 volleys. It doesn't take long to dismantle an enemy fleet in this manner.
