if you are using sequence number generators then they will not reset (you should be using them).

SQL> create sequence my_seq cache 20;

Sequence created.

SQL> 
SQL> select sequence_name, cache_size, last_number from user_sequences;

SEQUENCE_NAME                  CACHE_SIZE LAST_NUMBER
------------------------------ ---------- -----------
MY_SEQ                                 20           1

SQL> 
SQL> select my_seq.nextval from dual;

   NEXTVAL
----------
         1

SQL> 
SQL> select sequence_name, cache_size, last_number from user_sequences;

SEQUENCE_NAME                  CACHE_SIZE LAST_NUMBER
------------------------------ ---------- -----------
MY_SEQ                                 20          21

SQL> 
SQL> select my_seq.nextval from dual;

   NEXTVAL
----------
         2

SQL> 

If oracle is restarted, you loose the cached values.

Also, when using the sequence in your trigger you
don't have to select in into a variable first,
just use it in the sql.