@Tamamo: Since you piqued my interest about it, I took a look; While this mess is complicated to navigate, I'll try to disseminate what I'm looking at here.
I think it's 300 Coins for each Servant you'd wish to level to 120. Bond Levels 1-5 give 5 per level, 6-10 gives 10, 11-15 gives 15 - basically, getting a Servant to Bond 15 = 150 Coins, leaving you with 150 more to gain from how many copies you own, and that's dependent on rarity. 5* Servants give 90 Coins per NP level, so you'd need NP2 minimum there as well as Bond 13, as the math would then be 180 from NP levels + 120 from Bond levels to make the 300 needed for purely leveling to 120.
This doesn't take into account any of the Append Skills though, which also take Coins to level; 120 for each skill, so 360. Unless there's other factors I'm missing such as perhaps copies beyond NP5 granting more Coins, I don't know if you can level a Servant to 120 and max out all of their Append skills as well, since the 360 there for skills would work for a 5* Servant exactly IF you could account for NP6. Otherwise, there's a gap of 90 Coins short at NP5.
I guess tl;dr would be a minimum of NP2 and Bond 13 if I've read this right, I'll share the link to what I found here so anyone else can peep at it. No idea if I'm right here though, so I would welcome anyone that can correct or confirm this analysis.
https://gamepress.gg/grandorder/serv...hing-level-120
Addendum: This is just for the Coins that would be needed though; Other major factors would be the QP, Grails, and XP required to do this lol. 15M + 1 Grail for a 5* Servant to go from 100 to 102, so 150M and 10 Grails for 100 to 120, and then a grand total XP card count of 4,180 5* Ember cards of either the corresponding Class type or All type. :O