Can I use multiple RF433 modules at the same time?
There are a lot of 433MHz modules out there, so "RF433" can mean many different modules and so it's difficult to answer properly.

Some of those modules allow to tweak the frequency a little. If your module does this, it may be possible to put each car in a separate frequency.

Some other modules apply spread spectrum techniques to improve frequency usage, tough it isn't very common on 433MHz modules. If yours does, either FHSS or DSSS, then you can have "channels" that appear as noise to the other channels.

If you don't have those features, you'll have to apply some software protocol to avoid collisions. Be it some sort of TDMA-like or CSMA/CA-like scheme, you'll need two-way communications (some 433MHz modules are one-way only).

If you have the simplest of modules, single-frequency one-way only, it can possibly be done, if your baudrate is fast enough to allow for a lot of errors for your actual data rate. Even then, you'll have to be very smart and employ a lot of redundancy and robustness and probably some sort of FEC and random timing to make sure all receivers get their data, without error and in time.