print(adc_sum);
this prints a single character.
print('A'); will print an A.
print(65) will not print 65. it will print the ASCII character for 65.
I gets worse. assuming you ADC has a REF voltage of 3 VDC a gain one 1 and is 8 bits. If you input 3 volts you get 255.

Avoiding floats we can scale to millivolts

millivolts = (adc_counts * 3000UL)/ 255 (note 255* 3000 = 765000 and requires a long)

Now to display it.
First you must set your LCD to the Home position (or any other fixed position)
Then use atoi() or sprintf to format it to a buffer (array of char). then print it one char at a time.
better, but more wore if you have limited code space is as follows:

millivolts = 1234
the_char = millivolts % 10 // you get 4
millivolts = millivolts /10 // millivolts is now 123
the_char = the_char + '0' // now it is ASCII '4'
save in an buffer in reverse order.
Make a loop and repeat.

With a little more code you can print without using a buffer.