How many poison counters warrant an EDH loss?
It is 10 just like in a normal game. The only format that changes the number of poison counters needed to cause a player to lose a game is Two-Headed Giant.

704.5c If a player has ten or more poison counters, he or she loses the game. Ignore this rule in Two-Headed Giant games; see rule 704.5u instead.
704.5u In a Two-Headed Giant game, if a team has fifteen or more poison counters, that team loses the game. See rule 810, “Two-Headed Giant Variant.”