It has less to do with the actual number, and more to do with how the number is said or written.
Any time the number is "one", or a fraction with "one" in the numerator, the result is singular. This also applies to negatives. See Is -1 followed by a singular or plural noun?
- One apple
- 1 apple ("one apple")
- Half an apple
- One half of an apple
- 1/2 apple ("one half apple" or "half of an apple")
- 1/4 apple ("one quarter apple" or "one quarter of an apple" or "one fourth of an apple")
- -1 volt ("minus one volt", "negative one volt")
Any decimal number, including 1.0, is plural. See Should we use plural or singular for a fraction of a mile?
- 1.0 apples ("one point zero apples", "one point oh apples")
- 0.5 apples ("zero point five apples", "oh point five apples")
Fractions with numerators larger than one can be handled both ways. This also applies to percentages. The plural form is used for countable objects, and the singular form is used for non-countable objects. See Is two-thirds plural?
- 2/3 of the people are here. (We are counting people.)
- 2/3 of the soda was left over. (We are not counting soda.)
- 75% of the computers are broken. (countable)
- 75% of the rice was eaten. (not countable)
Complex and imaginary numbers:
Complex and imaginary numbers only appear in technical contexts. I can only think of examples with units, for example:
- 5.7+3.1j kΩ at 500 Hz
- -1.0+0.9j mV at 10 kHz
Note that engineers usually use "j" instead of "i" to avoid confusion with I, the symbol for current. Mathematicians use "i".
In technical contexts, quantities for should be written with numerals and units should be written with abbreviations, which do not take plural. So "5 V" is okay, but "five volts" is only okay in non-technical contexts.