Calculating wealth values from richness in QGIS

I may suggest a couple of workflow according to your QGIS version.

Personally I prefer Greater than frequency tool, which keeps me from reclassifying each of the input distribution maps.

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Classic approach (prior to QGIS 3.16)

This is the workflow what you have described.

1. Reclassify your distribution map (continuous values) into binary ([0 or 1]) raster. Several tools are available, such as Reclassify by table.
2. Add the maps using Raster Calculator.

QGIS 3.16

There is Greater than frequency tool in the QGIS Processing Toolbox > Raster Analysis. This tool outputs the frequency (number of times) the values of an input stack of rasters are greater than the value of a value raster. (I wish we could provide the threshold value as a constant, but it has to be a raster).

1. Create a constant raster by Create constant raster layer tool (in Processing Toolbox > Raster creation). In your case, the constant value needs to be Zero.
2. Start Greater than frequency and select the constant raster (all zero) as Input value raster. Also select all your 6 distribution maps in the Input raster layers.