Units are summoned by the sorcerers in the Barracks, and the sorcerer works most efficiently with batched orders and without a change of plans. 

Therefore, after you select the units to summon, send your command to a sorcerer by tapping the button 'Summon'. The sorcerer will then start the summoning and finish the job in a batch.

Interrupting an on-going summoning task will upset sorcerers who will keep half of the resource cost for unfinished units to compensate their work.