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The banishment spell requires "an item distasteful to the target" as a material component.
The spell can be upcasted to affect multiple creatures.
What happens when the different creatures you target are very different from one another, and thus have different distastes?
Example: a booger used as a distasteful item to a Priest of Cleanliness, but the spell is upcasted to also affect a Barf Monster, which does not care about boogers.
It should go without saying that it's moot if the caster is using an Arcane Focus; Banishment specifies neither that the object have a cost, nor that the object is consumed. So it's a perfectly legal spell to be cast with an Arcane/Holy Focus, and thus the spell works perfectly fine when upcast with a focus.
Otherwise...
It's a tricky ruling, but the description for Banishment's material component says "an item distasteful to the target", and upcasting the spell essentially means increasing the number of targets of the spell. So I think it would be necessary for the caster to hold the material components for both (or all) of the targets, not merely one of them.
First, don't take the specific material components for each spell too seriously; they are hold-overs from the early days of D&D, and are mostly there for flavor. (They are also, more often than not, jokes. Message is a copper wire...a phone line, get it? And don't get me started on the legumes for Gust of Wind.)
Mechanically, in 5th edition, they have been trumped by two classes of objects: the Component Pouch, and Spellcasting Focus (whether Arcane, Druidic, or Holy Symbol).
The Component Pouch basically lets you ignore those material components, as per the PHB Equipment. (Bolding added by me.)
A component pouch is a small, watertight leather belt pouch that has compartments to hold all the material components and other special items you need to cast your spells, except for those components that have a specific cost (as indicated in a spell's description).
The Spellcasting Focus mechanically works the same way as per the PHB Spellcasting.
A character can use a component pouch or a spellcasting focus (found in chapter 5, “Equipment”) in place of the components specified for a spell.
Jeremy Crawford, the only person on the Wizards payroll who is allowed to make official public rule clarifications, has backed that up via Twitter.
If you're using a spellcasting focus or a component pouch, free components are abstracted away.
So, since the component for Banishment doesn't list a gold piece value, it's covered.
RAW, you only need to worry about having those material components if you are without a Component Pouch or Spellcasting Focus. Since every 'full' casting class is given a focus or component pouch at character creation, the intent is they're abstracted away by default. So, the only time you'd really have to worry about it in RAW is if you have been stripped of all your equipment.
I can think of one edge case; a Good-aligned spellcaster with a Focus wanting to Banish a Good-aligned extra-planar creature. Then, depending on the flavor of the campaign, I could see a DM ruling that you'd need to obtain something distasteful to make it work, since the holy symbol or crystal or whatever probably isn't.
