I would add a couple more, say 9 minimum. I have had a group of this species (the Ember, Hyphessobrycon amandae, just to avoid any confusion) for many years, and find that they are better with more rather than fewer. They remain very small, and even 12 in a 10g would not be problematic if you are regular with water changes and have some live plants even if nothing more than floating. You could add some lower fish, one of the "dwarf" cory species, too if you like. I had a 10g set up for over a year with 9 pygmy corys, 9 Boraras brigittae (which are pretty much the same in size and biological impact as Embers) and even shrimp (accidental, they arrived in with the corys). This tank was an experiment of sorts, as it had no filter, no light (was in a south window), just a heater, and relied on the plants for filtration plus the weekly partial water change of half the tank. Fish were fine, and plants did well, though controlling algae with variable daylight is a challenge.
Byron.