I get you....but correct me if I am wrong. The more clones he made the less powerful they become. Also what is his range like, Ive forgotten. Oh well. Either way the author could add many restrictions to twice powers so we can't say why didnt he do this, I suppose.
There needed to be some tradegy on the villain's side. Someone not too popular (dabi, Toga) but not insignificant...someone cool n funny that ppl would remember, aha lets kill Twice. Something like that.
The thing is, other than each copy being weaker and Twice needing precise measurements in order to make copies, I don't recall any other restrictions. I'm sure there has to be a limit to how many copies he can make but I don't know what it is. Each copy of himself could make other copies. Like when they were fighting the Liberation Front, Twice made a staggering amount of copies that kept in turn making more copies. I do however remember him saying it was easier to copy himself than others.
On a slightly separate note, now that I look over my own comment, I'm surprised it wouldn't have occurred to the League to kidnap EraserHead during that training in the woods with the wild wild pussy cats. Once Twice copied him once he could always make replicas regardless of whether they actually had EraserHead in their possession. Having the use of Eraserhead's quirk would make the League extremely powerful.
I wonder if the author killed twice because he was too powerful. Since he was not under mental restrictions anymore it would be too easy to solve conflicts with him. He could literally make hundreds of clones that had the quirk of anyone he copied.
Can you imagine the destruction caused if he made thousands of Dabis and Shigarakis.
What would happen if they kidnapped Eraserhead and let Dabi clone him.When storming the gates of a city the League could nullify hero quirks with ease while running rampant with a bunch of other clones. Additionally, the real League Villains would never need to be in the real fight since the clones could be sent out.
I could imagine readers wondering why Twice didn't just use his powers for this thing or that thing. So to eliminate the problem the ultra powerful twice's character posses to the story, the author got rid of him.