a) I have multiple ImageMagick commands and I need to convert those multiple commands into one. I tried it by putting all the parameters in single command, but somehow it was not working and I had to discard it.
magick -density 300 cheque25.jpg -depth 8 -strip -background white -alpha off cheque25.png
magick convert cheque25.png -resize 150% res_cheque25.png
magick convert -brightness-contrast 10x30 res_cheque25.png b_res_cheque25.png
magick convert b_res_cheque25.png -threshold 45% bin_res_cheque25.png
b) Also, is there any chance that merged commands will give any different output than multiple single command?
Your ImageMagick syntax is not correct in several ways. In ImageMagick 7, you replace convert with magick. Also your input should come right after magick. ImageMagick 6 is forgiving of syntax, but ImageMagick 7 is not. See http://imagemagick.org/script/porting.php#cli
Try the following:
magick cheque25.jpg -depth 8 -strip -alpha off -background white -resize 150% -brightness-contrast 10x30 -threshold 45% -density 300 bin_res_cheque25.png
If that does not work, then provide a link to your input image, so others can test your commands and verify against mine.
The combined commands should give the same as a properly formatted set of commands, provided no syntax errors are present and settings are reset where needed and parenthesis processing is properly used when and where needed. I make no guarantees, since your set of commands is not using proper syntax.