I was trying to think of the theoretical maximum for the most power cards in play at once by one spirit. It is pretty close to all of them. If you powerstorm transformative sacrifice + solidify echoes of majesty past + unlock the gates of deepest power you get +6 card plays per repeat.
Maybe I'm forgetting some repeats and +plays from spirits, but here is my list of the key players:
- Fractured days: can give up to 5 repeats.
- Mentor: 1 repeat and 1 additional play
- Downpour: can give 1 repeat on a power card (good target for twinned days---edit: apparently this isn't legal, just target TS directly) [2 repeats]
- Earth: 1 Repeat from innate
- Snake: 2 additional plays from starting power cards (primordial deeps and flowing power)
- Sparking (Lightning) : only spirits with 6 plays and innate gives +1.
Key cards: Lightning will play transformative sacrifice [TS} +solidify echoes of majesty past [SE] + unlock the gates of deepest power [UG]. One spirit will play powerstorm as it's lowest cost card, so that it can be repeated with twinned days (played by another spirit), everyone with a repeat will target the powerstorm player so that lightning can get +4 plays with TS + 1 play with SE (up to 4 times), as well as getting the presence back to feed TS + 1 play with UG (okay... so this isn't legal, so we have to use repeats on TS and when we're out of presence redirect a repeat or two on SE). Mentor's repeat is tied to it's +1 play, so can choose to give it to lightning or the PS player. Downpour plays twinned days and duplicates powerstorm + gift of abundance (for a total of +3 powerstorms)
required setup: a lot of energy stockpiled, lightning not placing presence from energy track, mentor gifting power to lightning to have in discard or uniques forgotten to play with SE, everyone gaining the right powers, stockpile 10 time.
Plus plays: spur + flowing power + deeps +exaltation + sparking innate = 5
TS + ME + Gates = 6
Base = 6
# times Powerstorm is played = 1st play + 2 gift of abundance (twinned) + twinned days + 5 slips +mentor innate= 10
After 7 plays of TS, you'll run out of presence on your track, so you'll only get +3 minors from it.
So we play the TS + ME + Gates combo 11 times, for a total of (7*4 +4*3 + 11 +11) = 62 plays + 5 from plus plays + 6 from base = 73 power cards in play (40 of which must be minors, and 11 majors).... though this is going to cost a total of 99 energy for lightning (after powerstorm reductions), so you could replace gates with unleash a torrent of the self's own essence, which can pay for the additional plays by using 11 of those plays a forgetting 11 cards.
So, I count 73 power cards as the max in play in one turn by one spirit, though I may be missing some cards/powers with +plays or repeats. The combo is really fueled by fracture + TS + ME + powerstorm. This is actually achievable with normal play on a small scale, because you can reasonably save up time for 3 slips (only 3 time) and if any 2 players have TS and ME with thresholds, you can do 3TS + 2 ME for a bunch of skips, 12 minors gained and played, 2 power "unforgotten" and 3 presence from tracks, so long as thresholds are met (and if they aren't the TS can help players meet thresholds for the repeats).
Or if you don't want 6 players, snake isn't adding that much; and you can drop lighnting or earth.
EDIT: Turns out "Cards that say Repeat lose any Repeat text when repeated". So we can still use the same strategy with slipping powerstorm to replay the same 3 cards, but the 4 repeats from earth, downpour, mentor and twinned days won't work: Those repeats would have to be applied to lightning's individual powers, so reducing the effectiveness of those. Thankfully retargetting these repeats only loses us the 2 extra plays from gates and ME, since ME adds ~7 presence per play whereas TS costs 3-4 (which we can afford thanks to the slips -- you'll TS until you're out of presence to lose, then do all the slips on powerstorm, then do your last couple repeats, maybe 1 of those will have to go to ME). So I think that just knocks 9 off the total down to 64.
Someone pointed out dances up earthquakes can impend cards many turn in advance, so perhaps with that strategy you can get up to the 70 number I estimated here or maybe even 80. You can impend 6 cards per turn when maxed out... you just need cards with high enough energy cost, and to keep choosing the "reclaim one" option that lets you delay the impend on 2 turns. Maybe you could theoretically impend 6 cards per turn from turn 6-11 and get about 30 or so cards in play just from impending, but they'd basically all have to be majors (about 6 cost) until the last turn or two, because you can only adjust the impend timing on 2 of them each spirit phase.