A 247 If by "summon", you mean specifically "Conjuration (Summoning)" spells, the only thing I can think of is the Fiendish Summoning specialist feat in planar handbook, which adds an evil monster to each Summon Monster spell, but requires lots of DM cooperation. If calling spells are included, then there's malconvoker 8 (which adds 2 hd to the limits of planar binding spells) using greater planar binding, Infernal bargainer feat in Races of Faerun (requires outsider, also increases planar binding hd limits by 2), and if Dragon Magazine is open, Dragon 336 has the Implore spell which acts as Planar Binding, but up to 22/24 HD (it is inconsistent which it is, but definitely can get balors).