Gokaigers seem to change into past heroes one to five times per episode so far. The least being one (Red, Pink and Green became JAKQ in Ep 10) and the most five I think the most (Luka became a bunch in episode 6). But there are plenty of other examples but this is just one example. Most episodes the Gokaiger become more than one team but in some episodes, they just become one team and the other Gokaiger become one of different teams (Green and Yellow have done a lot of this).
Gokai Green has become Deka Green six times (2, 4, 5, 26, 35 and the Ghost Galleon movie). It previously was Shinken Green that he became the most.
Gokai Pink has also become a Dekaranger the most. She became Deka Pink six times (2, 4, 5, 26, 35 and the movie). She became Go-On Black the most (5 times: 3, 5, 19, 36 and the 199 heroes movie).
Gokai Blue has become Go-On Blue five times (5, 19, 36, 45, and the Gokai vs Gosei movie).
Gokai Silver has became Gold Mode more than anything but of the other Ranger Keys, he has become Shurikenger 4 times (21, 24, 43, 47).
As for the Ranger Keys, these are the keys that have been used the most:
Deka Red Key has been used six times by GokaiRed, once (but seen twice) by GokaiGreen (Goseiger Vs. Shinkenger and Ep40), and stageshows by GokaiYellow. Magi Red Key has been used by Gokai Red twice, Gokai Green in stageshows, by GokaiYellow in Episode 2 and stageshows; and by Gokai Pink in Episode 48 and 40 (Goseiger vs. Shinkenger). Magi Red Key is one of the keys that has been used by the most Gokaiger: 4. Geki Red Key has been used 4 times by GokaiRed (4, 7, 32 and 42); used by Gokai Yellow in 40 and by GokaiPink in 2. Go-On Red Key was used 4 times by GokaiRed and by GokaiYellow in stageshows. Shinken Red Key used 5 times by GokaiRed, and by GokaiYellow and GokaiPink in the stageshows.
As for the Sixth Ranger Keys, the Mega Silver key has been used seven times, seen in two episodes used by Basco; 3 times by GokaiSilver and twice by GokaiYellow. Gokai Pink used the Deka Pink Key six times. The Pink Keys haven't been used a lot by other Gokaigers like the Red or Sixth Keys, the only keys that have been used by two people have been Bouken Pink, Pink Flash, Ptera Ranger, Dyna Pink and Five Pink. Gokai Blue has become Go-On Blue five times. The blue keys also haven't been used alot. The keys that have been used by two people were Ao Ranger, Magi Blue, Blue Dolphin, Gosei Blue and Hurricane Blue (by the original owner!) As for the Extra Hero keys, all the keys have been used three times. Rio and Mele keys were used the least, they were only used once. Kuro Kishi Key has been used once by Basco, Gokai Silver used it once in his mecha and twice transformed into him, so it can be said it was used the most.
Deka Green key has been used 6 times by Gokai Green and the keys used by 2 people have been Shinken Green, Shishi Ranger, Dyna Green, and Green Flash. Shishi Ranger key has been used 6 times (5 by Green and once by Yellow). The Hurricane Yellow has been used six times by three different people, 4 times by Yellow, once by Green and once by the original user. Keys that have been used by 2 people are Gosei Yellow, Gao Yellow, Kirin Ranger, and Tiger Ranger. Go-On Black key is the black key that has been used the most by 1 gokaiger: 5 times by Pink. The black key to be used the most by 2 people is Ninja Black (4 times by Green and once by Yellow); another is Abare Black (once by Blue and once by Green). As for White keys, Deka Break and Kiba Ranger have been used 5 times (twice by Basco, twice by Gokai Silver and once by another Ranger.)