The Universal Approximation Theorem states: In the mathematical theory of artificial neural networks, the universal
approximation theorem states that a feed-forward network with a single
hidden layer containing a finite number of neurons can approximate
continuous functions on compact subsets of Rn, under mild assumptions
on the activation function. The theorem thus states that simple neural
networks can represent a wide variety of interesting functions when
given appropriate parameters; however, it does not touch upon the
algorithmic learnability of those parameters.
So clearly if you do not have infinite number of hidden neurons you cannot approximate a function with $\epsilon$ error tending to 0. So if we assume that the previous layer gives some sort of representation of the function to be approximated you still need infinite hidden neurons to approximate this representative function.
But probably it'll be more clear to you if we go into basic physical laws of mathematical approximation.
According to the Fourier Theorem by Joseph Fourier: A Fourier series is a way to
represent a function as the sum of simple sine waves. More formally,
it decomposes any periodic function or periodic signal into the
weighted sum of a (possibly infinite) set of simple oscillating
functions, namely sines and cosines (or, equivalently, complex
So theoretically, if your function is periodic it maybe perfectly approximated by an activation function like $sin$ or $cos$. The simple reason you cannot use an activation like $ReLu$ or $Sigmoid$ is because they are non-periodic and as per another related theorem called Fourier Transform you would require infinite number of sine waves to approximate non-periodic curves so the problem collapses in itself.
Since there are various ways to train a Neural Network other than Gradient Descent theoretically speaking you just might approximate a periodic function with some other methods (if activation is sine or cos), it is almost imposiible via methods like Gradient Descent.
So in short the answer to your question will be $\infty$ for the aforementioned reasons.