How many EXP Cards do you use?

I always get super success when my servant has 2 or 3 levels left to reach level max

The bulk one

All 20 if there's still a lot to go, in bulks of halves depending on the level requirement if it's close to max.

I always prioritize *4 exp cards to be spent. Normally, i'd save *3 exp cards to be used when my servants nearly reached max level cap but for having a great/super success, i often got it when i used *4 exp cards combined with *3 exp cards

It is rather risky to lvl up Servants with bulks of EXP. I prefer to use only one 4 Star EXP card everytime in order to raise the chances of Great/Super Successes.