Lemma 109.28.1. There exists a finite type morphism of algebraic spaces $Y \to X$ with $Y$ affine and $X$ quasi-separated, such that there does not exist an immersion $Y \to \mathbf{A}^ n_ X$ over $X$.

Proof. See discussion above. $\square$

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).