I think what ParamiteSurprise is referring to is the 28 original muds in RuptureFarms. You have to save them all before leaving the first Zulag and going to the Stockyards. There are a great many secret areas where muds are hiding. Try jumping down trapdoors or climbing down ledges hidden by foreground barrels.
If you can't find them all, use Google to find a walkthrough.
__________________
:
Spending as long as I do here, it's easy to forget that Oddworld has actual fans.
Okay, you need to be a bit clearer. Are you asking whether you need to start at the beginning of the game or start at the return to Zulag 1?
If so, you have to start from the start of the game to save all the muds. The 28 in the initial visit to RF are killed if you don't save them. That said, if you want to kill all muds you have to at least start at the Stockyards escape because there are muds there that remain as employees.
__________________
:
Spending as long as I do here, it's easy to forget that Oddworld has actual fans.