All 433MHz transmitters nominally transmit on the same channel. The receiver is as broad as barn door, so it will lock on to the strongest transmitter it hears, meaning it will likely be blocked from receiving any additional signals if at least one signal is already present. Worse, if it hears two or more signals simultaneously, then likely nothing can get decoded successfully.
You are essentially building a many-transmitter to a single-receiver system, so the only hope you have is give each transmitter its own time-slice using time-diversity, which means having it make a short-duration transmission at a puesdo-random interval to minimize the likelyhood of a collision with another transmitter. Unless you put a receiver at each sensor, then a given sensor cannot know that its mate is transmitting.
I would use a micro-controller at each transmitter to implement short duration transmission but at slightly different periods. I would modulate each transmitter with a different square-wave frequency, say 1kHz, 1.5kHz, 1.8kHz, 2.1kHz, ... and so on. Then connect a single receiver to the PI and write code to recognize the frequencies.
You could even allocate two tone frequencies to each sensor. That way, you can poll all the sensors to check that the sensors are all healthy (and within radio range) as well as that one or more of them is sending an alarm...
Alternatively, if you use micros that have a UART, hardware or software, have each blurp out one of two unique ASCII chars, repeated say five times each transmission. e.g. for sensor 1, A means sensor 1 is healthy, B means sensor 1 is alarming, sensor 2 use C and D, and so on. Use a slow baud rate, like 110baud per Roman. Repeat each character five times and have the Pi validate by receiving the expected char say three times in a row...
The goal is to maximize the probability that the receiver hears a single sensor at a time, without a collision...