Is there a maximum horizontal acceleration for passengers?
It depends on several factors:
1) How long is the body subjected to these forces?
2) What's the posture of the body?
3) To what extent are the passengers used to g forces (for example due to training)?
... 

However, early experiments showed that untrained humans could withstand g forces as much as 20 g for less than 10 seconds, to 10 g for 1 minute, and 6 g for 10 minutes.