Oracle 18c

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create table t(Item, Quantity) as 
  select 'Crankshaft', 200 from dual union all
  select 'Crankshaft', 100  from dual union all
  select 'Crankshaft', 200 from dual union all
  select 'Crankshaft', 50 from dual union all
  select 'Crankshaft', 200 from dual union all
  select 'Gear', 400 from dual union all
  select 'Gear', 200 from dual union all
  select 'Gear', 300 from dual union all
  select 'Gear', 400 from dual union all
  select 'Housing', 120 from dual union all
  select 'Housing', 50 from dual union all
  select 'Housing', 50 from dual
12 rows affected

select distinct
  item,
  max(quantity)
    keep(dense_rank first
    order by count(*) desc
  ) over(partition by item)
    as quantity_mode,
  max(count(*))
    over(partition by item)
    as mode_count
from t
group by item, quantity


ITEM
QUANTITY_MODE
MODE_COUNT


Crankshaft
200
3

Housing
50
2

Gear
400
2


select
  item,
  stats_mode(quantity) as quantity_mode,
  regexp_count(
    listagg (
      '%' || to_char(quantity, 'TM9') || '%')
      within group (order by 1)
  
    , '%' ||
      to_char(stats_mode(quantity), 'TM9')
      || '%'
  ) as mode_cnt
from t
group by item


ITEM
QUANTITY_MODE
MODE_CNT


Crankshaft
200
3

Gear
400
2

Housing
50
2