You can use the following expression to categorize your field values into any arbitrary number of percentile (n-tiles, so to say), like e.g. into 7 categories: when you have 140 features, the 20 features with the lowest values are the first category, the features with the next heigher 20 values category 2 etc.

The only thing you have to adapt is A) replace "fieldname" with the name of the field you use (line 18) and B) change the names and number of the output categories (line 3). Based on the number of elements you define there, the rest of the expression automatically calculates the right size of the categories. In the following example, I have 7 categories defined:

with_variable ('mp',
with_variable ('ar',
    array ('Very low','Low','Medium','High','Very High','Extreme','Very Extreme'),  -- change categories here
    hstore_to_map( 
        array_to_string(
            array_foreach (
                generate_series (0, array_length (@ar)),
                to_string (@element+1) || '=>'  || @ar [@element]
            ),delimiter:=','
        )
    )
),
map_get (
        @mp,
        array_length (map_avals (@mp))+1 - array_sum(
            array_foreach (
                map_akeys (@mp),
                "fieldname" <= array_sort (array_agg ("fieldname"))  -- adapt fieldname value twice here
                [array_length (array_agg(@name)) / array_length (map_avals (@mp)) * @element-1]
            )
        )
))

You can use the following expression to categorize your field values into any arbitrary number of percentile (n-tiles, so to say), like e.g. into 7 categories: when you have 140 features, the 20 features with the lowest values are the first category, the features with the next heigher 20 values category 2 etc.

The only thing you have to adapt is A) replace "fieldname" with the name of the field you use (line 18) and B) change the names and number of the output categories (line 3). Based on the number of elements you define there, the rest of the expression automatically calculates the right size of the categories. In the following example, I have 7 categories defined:

Running the expression on a layer with 99 features and 7 categories groups the 14 lowest values to the first category, the next 14 values to the second category etc.:

with_variable ('mp',
with_variable ('ar',
    array ('Very low','Low','Medium','High','Very High','Extreme','Very Extreme'),  -- change categories here
    hstore_to_map( 
        array_to_string(
            array_foreach (
                generate_series (0, array_length (@ar)),
                to_string (@element+1) || '=>'  || @ar [@element]
            ),delimiter:=','
        )
    )
),
map_get (
        @mp,
        array_length (map_avals (@mp))+1 - array_sum(
            array_foreach (
                map_akeys (@mp),
                "fieldname" <= array_sort (array_agg ("fieldname"))  -- adapt fieldname value twice here
                [array_length (array_agg(@name)) / array_length (map_avals (@mp)) * @element-1]
            )
        )
))