Summoning 10 Heroes at once is always more efficient than summoning 1 Hero due to the Gem discount - you save 400 Gems when doing 10 summons at once.

Note that the probability of receiving Epic or Legendary Heroes is the same for every single Summon, even if bought in a set of 10. It is not guaranteed that you'll receive an Epic or Legendary with a Summon x 10, but on the other hand, you may also get lucky and receive just the Hero you were after!

Please also note that the Elemental Summon has increased odds for 5* Heroes, so if you're after Heroes of specific Element it's usually recommended to use the Elemental Summon.