Question

+3 nova is max

But it can has other skills, for example

3light skills
3nova
3mastery



( I think )

yes...you can technically get a 2 sorc 3 nova orb, with 3 lm in rare, that can have 20fcr as well
or even a 3 lightning 3 lm 3 nova 20fcr orb...I'd play lotto before that though

i have a +2 cold, +3 blizzard, +3 energy shield, +3 another skill, minor life/mana, and some other small mod.

im pretty sure you "may" get a +2/+3nova/+3 es/+3 light mastery,

but on a terrible low odd...

No. Max staffmod is 3 different skills.

Sorc or elemental skills can roll as a affix, Whereas the staff mods can hold 3 different skills as you said.

2sorc/elemental aswell as 3 different sklls is possible on a rare orb.

A magic orb can get 3Elemental aswell as 3 different skills,

TLDR: +6 Nova is possible

Both of those examples only have 3 staff mods tho