My recollection from Season One was that we gave Melkor 20 Maiar followers in their pre-balrog angelic states. Arien defected before the destruction of the Lamps, and the others fell and became balrogs at that time. Half of the remaining balrogs were destroyed in the War to Begin All Wars with the fall of Utumno (some of these may have been ice demons, rather than fire demons, to demonstrate both aspects of his 'extremes of heat and cold'...but they're all dead now).
So, we should have about nine remaining balrogs (of the fire demon variety) at the moment. In Season 2, we showed them taking over Angband (that had been left under Sauron's command) and hunting elves in the woods for sport. We have established them as thugs.
Tolkien's final note on the subject was that there were 'at most' only ever 3-7 balrogs. Obviously, three balrog deaths appear in his writing in the published Silmarillion and Lord of the Rings: (Gothmog, the other balrog who falls in Gondolin, and Durin's Bane). So, that's the bare minimum. Any non-Durin's Bane balrogs remaining at the end of the First Age can be killed off in the War of Wrath. That was not described in any detail, and the main impressive fight is between Earendil and Ancalagon the Black. So, if we add balrogs, we need to add balrog deaths, too.
It's rather important that we don't make balrogs easy to kill. If we start adding a balrog death to all major battles, we reduce their terror. If we have a bunch of them ineffectively doing something, that reduces their power. So, the way the First Age battles are written, NO ONE kills a balrog until the Fall of Gondolin. And then we lose 2, including Gothmog. So, that needs to be a big deal, we need to build up to the story of how and why it finally happens in Gondolin when it didn't happen earlier [real life reason - Tolkien wrote that story when it was easy to kill balrogs]. We can add balrog kills (so far, we know they mortally wound Fëanor and they kill Fingon, but we could have other elf deaths in battle be attributed to balrogs). Otherwise, we run the risk of showing a balrog die every time we see a balrog fight, and..that shouldn't be the case.
So, IF we do not add any earlier balrog deaths, and we have 7 balrogs going into the First Age, two will die in Gondolin and four will die in the War of Wrath. Alternatively, we can lose some along the way, in the Battle of Sudden Flame, in the Unnumbered Tears, etc. But like I said, we seriously run the risk of making a balrog no different on screen than a troll - large and scary looking, and not so easy to kill, but nothing particularly terrifying about it.
The rule of thumb is that to kill a balrog, you need to die in the contest. And preferably, there is some sort of long drop involved. Every fighter should fear facing one. Gandalf the wizard wasn't afraid of much, but he was afraid of that. Sauron knows that the balrogs are more powerful than he is, and acts accordingly. I am not suggesting that a balrog can single-handedly take on an army, but they are really quite difficult to destroy. If we have to draw a comparison, they should be closer to dragons than trolls. Killable, sure, but not by just anyone.