Dispel Magic Only Ends Spells
A magical effect that is not the result of an ongoing spell cannot be dispelled.
Jeremy Crawford has issued several Twitter rulings to this effect.
In this Tweet:
Q: Can dispel magic undo a druid's wild shape
A: Dispel magic ends spells. Wild Shape isn't a spell.
and this one:
Q: Does Dispel Magic auto work vs Channel Divinity powers? Ie.. Paladin's Vow of Enmity magic effect.
A: Dispel magic ends spells. A Channel Divinity option like the paladin's Sacred Weapon isn't a spell.
and this series of Tweets:
Q: Does dispel magic effect an arcane ward?
A: Dispel magic ends spells. Arcane Ward isn't a spell
Q: Does that mean magical items cant be dispelled either? We've assumed that they could be disabled temporarily with dispel.
A: Dispel magic can end spells that come from a magic item, but it has no effect on the item itself.
Also note that Dispel Magic can only end spells with an ongoing duration. It cannot remove the lingering effects of an instantaneous spell. PHB P. 203:
Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.
The March 2016 Sage Advice Column covers Dispel Magic as well. It reiterates that Dispel magic cannot end magical effects that are not the result of a spell, such as a vampire's Charm effect. It also reiterates the rule about Dispel Magic not working on the results of a spell with an instantaneous effect. It adds that a readied Dispel Magic cannot be used in place of a Counterspell, though in some limited circumstances, it may be almost as good.
To answer your actual question, in this context, targeting a magical effect clearly means you can target just one specific ongoing spell effect instead of stripping everything on a particular creature.
No, it only ends the spell on a single target
We can extrapolate an answer based on how dispel magic interacts with spells like bless:
From the Sage Advice Compendium:
If dispel magic targets the magical effect from bless cast by a
cleric, does it remove the effect on all the targets? Dispel magic
ends a spell on one target. It doesn’t end the same spell on other
targets.
Since the conjure spells actually bring creatures into the Prime Material plane and in essence holds them there, a dispel magic will only end the magic on a single creature; the rest would still have the magic to keep them present.
Best Answer
The Sage Advice Compendium includes a ruling on the DM (not the player) deciding what is summoned with a Summon X spell, and also includes a ruling on dispelling such effects once created:
Notice here (backed up by Mearls as asked by @Christopher) that they are saying the creations of a Summon X spell can be ended by dispel magic or temporarily vanish inside antimagic field (because they ARE magic) individually, but that one cast of dispel magic does not destroy them ALL. Targeting the caster with dispel magic would do nothing because the spell is not affecting him as a target; he is simply channeling the spell.
Breaking concentration would be the fastest way to deal with multiple summoned creatures since dispel magic doesn't have an AoE. The aforementioned antimagic field would also be effective if the caster was caught inside, rendering his concentration spell non-functioning in its entirety until he stepped outside of the antimagic zone again, in which case the creatures would reappear.