Any number of players can play any number of riders on a single agenda, provided each is a different rider. If multiple players want to play the same rider, the player first in speaker-clockwise order has priority.
The relevant points from the rules reference:
Multiple action cards with the same name cannot be played during a single timing window to affect the same units or game effect. Canceled cards do not count as being played.
During the strategy and agenda phases, if there are multiple abilities
that players wish to resolve at the same time, each player takes a
turn resolving an ability starting with the speaker and proceeding
clockwise. This process continues until each player has resolved all
the abilities that he wishes to resolve during that timing window.
There is nothing in the rules reference that prohibits a player from playing multiple riders specifically. There is also nothing besides the above rule preventing several action cards being played in response to a single trigger.