MP and MB are 2 different things.
MP, or Mega Pixels are the pixel dimensions of the image.
MB, or Mega Bytes, is a data file size.
A 5.4 MP JPEG image can be a wide variety of file sizes that depend on how much the image file is compressed.
Your JPEG images was compressed at least once already, and your computer will say how many MB the file currently is there on your computer.
Now.
When a digital image file is made into a physical document, like a magazine cover, another number comes into play, and that is the print resolution.
Print resolution is specified as so many PPI or pixels per inch.
If your JPEG was made on a DSLR camera and hasn't been cropped, it likely has a 3:2 aspect ratio and is just under 3000 pixels by 2000 pixels (6 MP)
Most monthly magazines are close to 8 1/2 x 11.
So 3000 pixels divided by 11 inches = 272.7 PPI which should be plenty.
If your JPEG file is smaller than 12 MB, you can increase the the image resolution which will also increase the file size.
But, increasing the resolution means making up pixels that never actually existed, and if not done well can f'up your photo such that the magazine cannot use it.
So tell us what the pixel dimensions and file size of the JPEG are.
FWIW.
The average use licensing price for one time, non exclusive reproduction rights of an image to be used on the cover of a magazine:
that has a print run of 10,000 or fewer magazines is - $775
that has a print run of 10,000 to 50,000 magazines is - $975
that has a print run of 50,000 to 100,000 magazines is - $1050
that has a print run of 100,000 to 250,000 magazines is - $1100
that has a print run of 250,000 to 500,000 magazines is - $1200
that has a print run of 500,000 to 1M magazines is - $1375
that has a print run of 1M to 3M magazines is - $1500
that has a print run of 5M to 10M magazines is - $1850
that has a print run of over 10M magazines is - $2250