How to multiply a .txt data file by a constant number?

I have a .txt file with one column. it is about 8000 numbers. How can I multiply this column of data by 1000000?

You can use awk command:

awk '{print $1*1000000}' file.txt

If the numbers in the file are integers or simple floating-point values, you could use the numfmt utility with --from-unit= to indicate the desired scaling.

Ex. given

$ cat file
1.23
5
3.45
17
6.78
23

then

$ numfmt --from-unit=100000 < file
123000.00
500000
345000.00
1700000
678000.00
2300000

You can add a variety of printf-style formating to the output e.g.

$ numfmt --from-unit=100000  --format="%'12.2f" < file
  123,000.00
  500,000.00
  345,000.00
1,700,000.00
  678,000.00
2,300,000.00

Alternatively, with sed and bc:

sed 's/$/ * 100000/' file | bc

or (reverse polish variant)

sed 's/$/ 100000 * p/' file | dc