Bigquery,SQL, how to calculate number of unique value over a row

Let' say I have a table like this:

I want to create a new column which counts the number of unique value over each row in columns math,english,science;
So this is my expected output:

For the first row, there are only two kind of score 69, 70 so n_unique is 2,
for the second row, there are 65,71,63 so n_unique is 3,
for the third row, only one score 66, so n_unique is 1;
How to write the query to create such column in Bigquery using SQL?

Consider below approach

select *, (
    select count(distinct val)
    from unnest(regexp_extract_all(format('%t', t), r'\d+')) val
  ) as n_unique
from your_table t     

if applied to sample data in your question - output is

You can "unpivot" your table, count the distinct grades per student, and then join back to your original table:

with mytable as (
  select 'Amy' as name, 69 as math, 70 as english, 70 as science union all
  select 'Mike', 65, 71, 63 union all
  select 'Jay', 66, 66, 66
),
tmp_unpivot as (
  select * from mytable
  unpivot(grade for class in(math, english, science))
), 
agg as (
  select name, count(distinct grade) as n_unique
  from tmp_unpivot
  group by 1
)
select
  mytable.*,
  agg.n_unique
from mytable
inner join agg on mytable.name = agg.name