Finding overlapping areas in same polygon layer using QGIS?

QGIS 3.4

1. Make sure your polygon layer has a unique id field, such as id or fid.
2. Run Union tool (Processing Toolbox > Vector overlay). It will return a new Union layer.
3. Open the attribute table of this Union layer, and create an integer field with an expression as below: count("id", group_by:=geom_to_wkt($geometry)) ...... if your id field name is "id".

In Database/DB Manager/Virtual layers/Qgis layers you're able to execute the following query (you'll have to adapt the column names...) :

with a as (select * from buffer_table)
select to_real(buffer_table.id) id, count(*) nb from buffer_table,a
where st_intersects(buffer_table.geometry, a.geometry) and a.id<>buffer_table.id
group by buffer_table.id

You load this table and save it in QGIS and join the result to your buffer_table and then you can use the nb column to label or style your buffer objects.

Try this: Processing toolbox - SAGA self intersection your result will like 1|5|7 and so one

via DB manger/SQL you can make a query to "count" '|' as indicator for the number of overlaps:

Create table test_count3 as select length("ID")-length(replace("ID",'|','')) as test, ogc_fid FROM test_split END

join this layer to your self intersection layer via ogc_fid and style it as categorized with the "test" column (But I am sure there is a more elegant way, however it seems to work...)