Should I use is or are in this sentence?
100 apples are/is considered as a large number of apples.
Subjects expressing periods of time, amounts of money, or quantities may take either singular or plural verbs depending on whether [they] represent a total amount or a number of individual units. For example, "Four weeks is not enough vacation time" and "Two days have passed since I asked for your response." (Section 2 paragraph 8 of source)
In your example, as the complement ("a large number") is singular, the subject represents a total amount, and so is treated as a singular. I would use is.
Compare that with
100 apples are rolling down the hill.
Here the 100 apples represents 100 individual units, so I use the plural.
The source notes that this is a tricky point, and there is variation among native speakers.
Rephrasing can avoid this issue:
One hundred is considered to be a large number of apples
We consider a hundred to be a large number of apples.
Both can be considered correct, but I think there are good reasons to prefer "is".
If you say "100 apples are [something]", there's an expectation that what you're saying applies to each apple individually. For example, if I tell you "ten children are playing football", you'd expect me to be able to justify my claim by pointing at each of the children and saying, "That child is playing football." However, if I say "100 apples are a large number", I can't point to any single apple and say "That apple is a large number."
Instead, you're implicitly talking about a single collection of apples. The collection has the property of being a large number, but the individual apples don't.
One hundred is considered a large number of apples.
One hundred apples is considered a large number of apples.
One hundred would be considered a large number of apples.
Here are some examples of how a plural noun has nothing to do with verb agreement when the subject is a single entity:
100 apples is a large number of apples. (it's a singular thing called "100 apples")
That barrel of apples is full. (Barrel is singular)
The number of apples you have is large. (Number is singular)
The amount of apples in the warehouse is large. (Amount is singular)
The 100 apples you have there are in a large pile. (100 separate things are...)
100 is a large number (it's not 100 "ones," it's one thing named "100"). The number 100 is large... (again, a number is singular, no matter how high the count).
The orchestra (it) sounds great. Its members (the players) play magnificently.
Well, it's a horrible sentence for all sorts of reasons:
You don’t consider apples in isolation, you consider them with some goal or quality in mind. Try:
“We don't need 100 apples.”
When a sentence sounds that awkward, it’s usually best to rethink it from scratch.