Unknown error in assembly language code, the result of multiplication operation is always random symbol

I have just started learning assembly language and i tried to write a program on my own with notepad++ and dosbox emulator for multiplication operations. However, the result is always random symbol that doesnt tally with my expected answer.

Here is the full code I've written

.model small
.stack
.data
    msg1 DB "Quantity (unit): $"
    msg2 DB 13,10,"Unit price (RM): $"
    msg3 DB 13,10,"Total amount is RM$"
    Quantity DB 0
    Unit_price DB 0
    Total DB 0
    
.code
main proc
    mov AX,@data
    mov DS, AX
    
    mov AH, 09h           ;display 1st msg 
    lea DX, msg1
    int 21h
    mov AH, 01h
    mov Quantity, AL
    int 21h
    
    mov AH, 09h           ;display 2nd msg
    lea DX, msg2
    int 21h
    mov AH, 01h
    mov Unit_price, AL
    int 21h
    
    mov AX, 0
    
    sub Quantity, '0'
    sub Unit_price, '0'
    
    mov AL, Quantity
    mul Unit_price
    add Total, AL
    add Total, '0'
    
    mov AH, 09h
    lea DX, msg3
    int 21h
    
    mov AH, 02h
    mov DL, Total
    int 21h
    
    mov AX, 4C00H
    int 21h
    
MAIN ENDP
END MAIN

The picture is the result i obtained from multiplication of 2 and 3 (should be 6 but idk why it's an arrow)

You wrote:

    mov AH, 01h
    mov Quantity, AL
    int 21h

The character that was read is returned in AL by the int 21h service, so mov Quantity, AL needs to come after int 21h.

As noted by fuz, adding/subtracting '0' to convert a number to/from ASCII only works for single-digit numbers. So your program should work for calculating 2*3 as in your test, but not for 3*5; you'd have to do a proper decimal conversion for that (repeatedly multiplying/dividing by 10 to extract digits one by one).