Do I have to use the same amount of digits or could I just enter "12345"? Or a word?
If you compile the program in $/Tools/QtBiomeVisualiser (it requires Qt 5+ libraries), it will render the biomes for any world.ini.
The seed can be any number within the valid range, I'd suggest using a positive one, though. Any text will be accepted, but will be treated as "seed=0".