The numbers indicate the cumulative maximum theoretical bandwidth for all the devices in the package.

The Bands Available and their maximum theoretical bandwidths are 2.4 Ghz: 480 Mbps 5 Ghz: 1300 Mbps 6 Ghz: 1300 Mbps [not sure about this]

For a single device:

AX1800 2.4 + 5: 480+1300 ~ 1800 Mbps

AX3000 2.4 + 5 + 5: 480+1300+1300 ~ 3000 Mbps

For Multiple Device packs: If you get a mesh system that has n devices, then you take the AX Number and divide it by n.

So a Mesh system with 3 devices termed as AX5400 is actually 3 individual AX1800 devices.

Now on Wifi6, if your router supports MIMO and OFDMA [fancy way of saying a router splits the data transfer of one wide band into a certain number of smaller bands simultaneously]

So an AX5400

MIMO 2x2 on 2.4 Ghz ~ 300x2 = 600 Mbps

MIMO 4x4 on 5 Ghz ~ 1200x4 = 4800 Mbps

Sources: https://www.mwrf.com/markets/article/21846554/understanding-benefits-of-mimo-technology