Approach 1 (completes quickly but randomly distributes as customer_no is not a sequence generated num. The range will not help me in getting equal distribution):

v_no_of_sets 	      number := 5;
recs_per_set        number;
v_cnt                 number := 1;
v_init_cnt            number := 1;


select max(to_number(customer_no)) into v_max_customer_no from customer_trans;

select min(to_number(customer_no)) into v_min_customer_no from customer_trans;

recs_per_set := ceil((v_max_customer_no - v_min_customer_no)/v_no_of_sets);

while v_init_cnt <= v_no_of_threads loop    
    update customer_trans
    set set_id = 100+v_cnt
    where to_number(subscriber_number) between (v_max_customer_no - recs_per_set) and v_max_customer_no;
    
    commit;    
    
    v_cnt := v_cnt + 0.01;
    v_init_cnt := v_init_cnt + 1;
    v_max_customer_no := v_max_customer_no - recs_per_set - 1;
end loop;
commit;

Approach 2 (Using cursor which gives proper result but takes more time for each set):


v_cnt                   number;
v_recs_per_thread       number;
v_run_id_cnt            number := 1;
v_hardcode_run_id       number;
v_track                 number;
v_abs_track             number := 0;
v_max_thread            number := 1;
v_no_of_threads         number := 5;

cursor c1 is
select customer_no, count(1) 
from customer_trans
group by customer_no;

select count(1) 
into v_cnt
from customer_trans

v_recs_per_thread := ceil(v_cnt/v_no_of_threads);  

for i in c1 loop
    v_hardcode_run_id := 100; 
    v_abs_track := v_abs_track + i.sbscrp_cnt;            
    
    if v_abs_track <= v_recs_per_thread then        
        update customer_trans
        set nxpp_run_id = v_hardcode_run_id + v_run_id_cnt   
        where customer_no = i.customer_no;        
    else
        v_max_thread := v_max_thread + 1;        
        if v_max_thread <= v_no_of_threads then       
            v_run_id_cnt := v_run_id_cnt + 0.01;            
        end if;

        update customer_trans
        set set_id = v_hardcode_run_id+v_run_id_cnt
        where customer_no = i.customer_no;                

        v_abs_track := i.sbscrp_cnt;            
    end if;
    
    IF MOD(c1%rowcount, 500) = 0 THEN
	     COMMIT;
	END IF;                       
end loop;