1) Your radios do not work at 433.0000MHz; 433MHz is the "band".
2) After selecting a band, radios select a "channel" within the band; i.e. 433.075MHz, 433.100MHz, etc.
3) After selecting a specific frequency, the radio modulates up and down from it. The difference between the beginning of channel "1" and the beginning of channel "2" is 0.025MHz; that is the "band-width".
So, selecting 433MHz band, channel "1", the radios occupy many frequencies between:
(433.075 - (0.025 / 2)) to (433.075 + (0.025 / 2))
433.0625MHz to 433.0875MHz
Check out the "Channels" table: http://en.wikipedia.org/wiki/LPD433
On the ISM bands (license-free), you are required to accept all interference, but are "encouraged" to produce none. If you imagine yourself and a friend trying to communicate verbally over a long distance, you know you'll need to talk loud. If other people are nearby and also talking loudly (interfering with your conversation), you'll have to talk even louder (interfering with their conversation), or find somewhere else to talk (change channels). Radio's are very much the same and many automatically negotiate the "quietest" channel to use. If it gets noisy, or communication becomes error-prone, they agree on a new (quieter) channel, and switch.
If you have a relatively quiet channel, all to yourself, you can run as many radios on it as you want, if you have some type of "flow-control" to keep the multiple radios from talking while others are. This can be as simple as assigning each radio an address (1, 2, 3, etc.) and preceeding all communications with a descriptor indicating who the recipient is. Radios with different addresses ignore the remainder of the packet. When it gets quiet again, all radios begin waiting for a new address byte.
1) The available bandwidth is 25kHz per channel, however, channels aren't really "real". You can transmit wherever you want within the ISM band. This IC can use 4kHz channel spacing, alowing you to transmit in a "sub-channel" of a channel of the ISM band. Cheaper ones just use the 25kHz (+-12.5). Lower bandwidth means exactly that, less bandwidth available for use. For example, this IC advertises that while using 12.5-kHz "channels", approximately 9.6kbps can be transferred.
2) Using the 4kHz spacing indicated by the datasheet, you could use approximately 6 distinct "sub-channels" (6 conversations at a time, using 1 channel). The total bandwidth of the channel will not change, but you can slice it up however you like.
3) The datasheet indicates the device does have some type of automatic low-power receive mode. In order to receive at all, the base-band block must be active. In this mode, the radio can only detect "something is being transmitted", not what, or by who, or to who. The first byte is often lost or corrupted; to "poke" the radios first, start the transmission with one or more null bytes (0x00). This makes sure all radios are ready to receive the ID byte. Using an 8-bit ID, with 0x0 reserved for "poking", 0xFF reserved for "all-call", 254 possible ID's remain. Using a 16-bit ID, many thousands of ID's are possible. Any radio that receives an ID other than it's own, goes back to sleep. You can Google many open-source stacks/shells/protocols to use instead of having to write this by hand.
4) There are a number of technologies to address this. RSSI indicates the ambient signal level on the "sub-channel" that you are using and all radios using "channel 1" will all see different RSSI values for their respective (narrower) sub-channel. This IC features "Clear Channel Assessment (CCA) for Listen-Before-Talk (LBT)". Essentially, the radios will wait for RSSI to be low (quiet) before they talk. This IC indicates support for "Retransmissions" and "Auto-Acknowledge of Received Packets", so, if two radios do happen to try to talk over each other, the radios will automatically make sure the messages get where they were going (without you having to detect the collision and manually re-send).
Below is a diagram of [sub]channels; this concept applies when selecting one channel out of a band, and selecting a "sub-channel" within that channel: