Good day jayl.jl and welcome to RP!
If you don't mind, please describe your setup with more detail, as the word "complete" is far too subjective. It would be easier to give livestock suggestions when it is based on the actual setup, not by the volume of the display tank.
Depending on how good your overall tank husbandry is, I say you can keep up to five fish (four optimally), provided that they are relatively small(max size ~3in) and non-territorial or really active swimmers.
Here is a list of some of the fish appropriate for your tank's capacity.
Easily obtained and cared for:
[*]Ocellaris Clownfish
[*]Red firefish
[*]Damselfish belonging to the genus
Chrysipterae.g. Blue damsel, yellow-tail blue damsel
[*]Small gobies (particularly the prawn gobies)
e.g. yellow watchman goby, clown goby
[*]Pajama cardinalfish
Some uncommon ones:
Purple firefish
Randall's goby, hector's goby, etc.
Tail-spot blenny
Possum wrasse
(depending on the source and supply of these, they may be just as pricey as the next group of fish)
Some occasionally available and, surely enough, are quite pricey:
Royal gramma basslet
Banggai cardinalfish
Helfrichi firefish
Starcki damselfish
Swalesi basslet
Designer clownfish (both ocellaris and percula)
Do take note of each fish species' special care requirements before making a purchase.
I suggested
Chrysiptera damselfish, based on my experience as well as from a friend's, because they show off the best coloration of all damselfish and are the least aggressive. My experiences are with blue damsel and yellow-tail damsel in a 50-gallon tank, which may have provided enough space for them to become mild-mannered. In a small tank, temperaments may vary, so do take caution if ever these fish catch your eye.
Also, take note that the streamline/torpedo-shaped fish (dartfish/firefish, gobies, basslets, wrasse) are accomplished jumpers, so do provide a cover for your tank.
The mandarin is definitely not recommended for that tank size because as bro Jeff (dwarven) has noted it requires a constant amount of live food in the form of copepods and amphipods. Although it is possible to wean it onto frozen foods but would take a lot of patience and effort.
All info I have noted is mostly based on personal experience, as well as from fellow reefers. So I hope this helps and good luck on your journey!