Question

why wont this give a mono-substitured product?

ta AICI3

what does it give multile products?

Answer 1

Good question

The reaction is known as Friedel Craft alkylation reaction wherein the AlCl3 acts as catalyst for formation of alkylated product.
During the reaction, there is formation of carbocation as intermediate.
Since in aromatic system, due to resonance, the stability of intermediate carbo cation increases, it also results into increase in number of attacking sites on aromatic ring.
Therefore, the electrophile can attack 3 different positions in aromatic ring resulting into formation of multiple products.

Thanks for posting.