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:

Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend.

[...]

In contrast, a spell like conjure woodland beings has a non-instantaneous duration, which means its creations can be ended by dispel magic and they temporarily disappear within an antimagic field.

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.

I play a druid character who regularly uses conjuration spells. To make life easy on my DM I prepared a cheat sheet of all the summonable subjects. It's just a little sheet that has AC, relevant abilities and a hit point marker for each summoned creature. If there are 8 creatures summoned for that CR, then there are 8 hit point markers. He usually has me run them for him, but he still makes the decision. Each entry on the sheet has key words that can help him make a quick judgement call: underground, flying, darkvision, poison, etc.

When I cast the spell, I tell him what my character is hoping will appear, usually using a keyword. If I am hoping for air support, I might get owls, or bats. For poison, I might get snakes or spiders. Part of the fun of D&D is dealing with surprise results (one of the main reasons the game has dice).

So yeah, if your players intend on making a lot of use of these spells, some prep work ahead of time can be helpful. If you can convince them to do the heavy lifting (as I did for my DM) so much the better.

In general, the DM should make something appear that has some relevancy and helpfulness for the current situation. The player is expending a spell resource, after all.