Tigli's Marvelous Menagerie

You summon 1d20 + your spellcasting ability modifier fey spirits that take the form of tiny beasts of Challenge Rating 0 and appear in a space that you can see within range, this space can coincide with a living creature. Sample creatures can be found below.

The summoned creatures are friendly to you and your companions. They can take commands telepathically when they are within range with no action(allowing the player to control the swarm), or as long as they can hear the caster shout the commands(GM determines actions) If they cannot receive commands they will follow the last command given, or start acting naturally if no command was given last turn (GM determines actions). 

A group of 20 or more become a large swarm, taking up 10ft x 10ft, a group of 100 or more become a huge swarm, taking up 15ft x 15ft

In Combat
Each beast is considered fey, and have 1 hit point. In combat a swarm is considered one creature, with their hit points combined i.e 4 Frogs have 4 hit points, but a group of 4 Crabs have 8 hit points
If a summoned beast cannot attack it receives no bonus to attacks from the beast's stat block, however it should be assumed damage is dealt via the impact of the swarm against the victim.

The swarm will attack any creature that is in its space as an action. Additionally the swarm will attack any creature that ends its turn within the swarm.
A swarm does 1d6 damage for every 10 beasts, and an additional 1d4 for any remainder.
Any saving throws inflicted by the swarm use your Spell Save DC instead of the summoned creatures, and only need to be rolled once per attack. When attacking with the swarm add your spellcasting ability modifier (not your spell attack) to the creatures to hit modifier. For all other rolls use the creatures stat block. Damage from saving throws is automatically max.
i.e. if a swarm of 23 Scorpions hits a creature it deals 2d6 + 1d4 piercing damage, however the creature only makes one CON save vs your Spell Save DC and takes 8 poison damage (or 4 on a successful save) only once

 Sample Creatures

At Higher Levels for every level above first add another 2d20 to the total number of summoned beasts. i.e. at 3rd level roll 5d20+ your spellcasting ability modifier