As I understand Ars Goethia (RoP:I) you can 1. Summon a proper spirit, 2. Bind it to yourself for Affinity bonus, 3. Summon another proper spirit, 4. Command the second one to teach you, 5..., 6. PROFIT
Based on that this logic is true what kind of spirits do you summon and use as a teacher?
My candidates are:
Roman Centurion as a source of REAL Latin language, Leadership and Single Weapon
Spirit of a semi-famous craftman for learning the particular craft
Some folk witch for Magic Lore
Some famous writer/philosopher for Artes Liberales and Philosophie (maybe Teaching)
The problem I see here is knowing of and having ACs to so many spirits, at least two with high levels in any ability you want to learn since the first spirit’s affinity-like benefit thing is capped at their level in that ability.
EDIT: Finding even one suitable pair of spirits is likely an adventure or two.
I prefer Hermetic Sahirs and their ability to persuade Jinn to teach them Realm Lore, Sihr (the first two Solomonic Sahir do as well) and the Hermetic Art most closely linked to their form. Keep summoning elemental Jinn for the win!
Back to the original point, I believe the classic use is to get magi of different ancient or hedge traditions and persuade them to teach you magic...whether it's more magic or faerie lore, or their theory, or Insights for Integration. Finding a great leader early on (to teach you Leadership, as the number of spirits you control at one time is limited by Leadership) is a great help.
There is no general rule for how many spirits one cn control at a time - it all depends on how you summon and control them.
For Hermetic Sihr, HoH:S p136
For Solomonic Sihr, TC&TC p38
If you use Elemental Summoning, the summoning itself costs a longterm Fatigue level which you won't recover until you release the summoned being. HM:RE p23
For the Goetic Arts, I can't find a listed limit to how many spirits you can control at the same time.
Same for spirits summoned and controlled using the Hermetic Arts - no limit listed.
HoH: Societates p136 - "A sahir can have a number of jinn in service to him at any one time equal to his Leadership score, although all sahirs can command at least one jinni."
as ErikT points out, it's the same for Solomic Sahir.
However, seeing as Leadership in the lab is limited to one assistant per point of Leadership and number who help the leader in a trained group is also limited to one per point of Leadership, I've always taken it as "One per point of Leadership can help" in anything important.
See ArM5 core rules p103 for help in the lab and p173 for trained groups in combat.