There's usually three or four vanguard systems per constellation, depending on the size of the constellation. In some of the larger constellations you can have up to six vanguard systems, but they are exceedingly rare. Each system will have up to seven vanguard sites spawned at once and there are three different kind of vanguard sites:

VG fleet alterations and notes

logi in vanguard sites do not cap transfer
fleet ships in these sites do not move
a sniper heavy comp is used meaning 70% sniper(e.g.nightmare) and 30% closerange(e.g.vindicator)
4 logi are used in high amounts of influence, and is lowered to the standard 3 after 25% influence remains