MariaDB 10.2

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create table t as 
    select 12 as CustomerID, 'Argentina' as country union all
    select 54, 'Argentina' union all
    select 20, 'Austria' as country union all
    select 59, 'Austria' as country union all
    select 5, 'Belgium' as country union all
    select 76, 'Belgium' as country union all
    select 77, 'Belgium' as country union all
    select 15, 'Brazil' as country union all
    select 21, 'Brazil' as country union all
    select 31, 'Brazil' as country union all
    select 34, 'Brazil' as country union all
    select 88, 'Brazil'
select * from t


CustomerID
country


12
Argentina

54
Argentina

20
Austria

59
Austria

5
Belgium

76
Belgium

77
Belgium

15
Brazil

21
Brazil

31
Brazil

34
Brazil

88
Brazil


select t.*,
       @p := (case when @c = country
                   then (case when @rn = 10  -- a county has more than 10 rows
                              then (case when (@rn := 1) = null then null -- never happens
                                         else (@p := @p + 1) 
                                    end)
                              else (case when (@rn := @rn + 1) = null then null -- never happens
                                         else @p
                                    end)
                         end)
                    when (@rn + cnt > 10)  -- start a new page
                    then (case when (@rn := 1) = null then null -- never happens
                               else (@p := @p + 1)
                          end)
                    else (case when (@rn := @rn + 1) = null then null -- never happens
                               else @p
                          end)
             end) as page
from (select t.*, c.cnt
      from t join
           (select country, count(*) as cnt
            from t
            group by country
           ) c
           on t.country = c.country
      order by t.country, t.CustomerID
    ) t cross join
    (select @rn := 0, @p := 0, @c := '') params
order by page, t.country, t.CustomerId


CustomerID
country
cnt
page


12
Argentina
2
0

54
Argentina
2
0

20
Austria
2
0

59
Austria
2
0

5
Belgium
3
0

76
Belgium
3
0

77
Belgium
3
0

15
Brazil
5
1

21
Brazil
5
1

31
Brazil
5
1

34
Brazil
5
1

88
Brazil
5
1
CustomerID	country
12	Argentina
54	Argentina
20	Austria
59	Austria
5	Belgium
76	Belgium
77	Belgium
15	Brazil
21	Brazil
31	Brazil
34	Brazil
88	Brazil