It is shared.
The best situation is up to 300 mbit/sec total. (e.g. 1 AP, 1 client). In practise it is a lot less because there is a lot of overhead in wireless and because speed drops rapidly if you move away from the AP.
Expect this to get significantly worse when there are multiple clients.
1 AP, 1 client, expect 300 mbit/sec total (real world in a nearby room: 10-15 MB/sec)
1 AP, 2 clients, (both active). Both maybe 5 MB/sec due to lots of retransmissions.
1 AP, 3 clients, ... even worse.
If you do not use wireless all the time (e.g. browse. download a page, read it while WiFi is idle) this works fine. If you have multiple computers running torrents or similar then you really do not want WiFi.
Post edit your post:
For example if there were 10 devices all trying to saturate the bandwidth,
would they all be able to do 54Mbit/sec or would each get around 5.4Mbit/sec?
Best case each would get 5.4mbit.
Likely case: A lot less.
Note that wired does have cumulative throughput though most switches.
Computer A ----- S A can communicate at 100mbit/sec to B
w At the same time
Computer B ----- i C can communicate at 100mbit/sec to D
Computer C ----- c If A tries to communicate at 100mbit to both C and D
h then speed will be shared
Computer D -----/