its not really a matter for how many of fish, it depends more on how large the adult size of the fishes and what type of fish (for compatability)
i agree with the tang probably outgrowing (i've read that tangs in general need a lot of growing space AND swimming space since they are not territorial fish and are nomadic)
and i was told 1 inch of adult size fish per gallon (about 2 liters) but this is different for different fishes because of swimming space and other needs
2 X Cromis
(I believe these are shoaling fish and like to be in big groups of 4+ or single/pair, BUT i could be wrong)
1 X Clown (nemo)
(a pair is nice to have, but one will be fine if you only want one)
1 X Regal tang
(as stated above)
1 X Royal Gramma
(I heard these fish can be a bit agressive but i never had agression issues)
1 X Dottyback
(same as Gramma)
2 X Cleaner shrimp
(sure why not)
Many hermets
(don't forget the shells)
A few snails
(try to get a good variety)
25kg live rock
( )
Loads of soft corral, a couple of little bits of hard corral.
(don't forget to watch you calc and other stuff with hard corals, softies are easy =])
You could try:
yellow watchman goby, wheeler goby, clown goby, hector goby, pink-spot watchman goby, pink bar goby
midas blenny, tail-spot blenny, One Spot Blenny, bicolor blenny
firefish
reef-safe wrasses (carpenter flasher, 6 line, fairy, mccosker flasher)
pajama cardinals
other types of clowns (like black and white, skunk)
yellow tail damsel (this is one of the more peaceful damsels but should still be added as one of your last fish)
im not really sure how many fish you could have, i only have 8 small fish in my 55 gal, and it looks empty and understock but under is better then over, depends on what fish
Hope, this helps, and sorry if i got something wrong, i'm sure someone will fix me