You mean a number with more than 38 digits?
You can insert numbers larger than this. But you only get 38 digits of precision:
drop table t purge;
create table t (
x int
);
insert into t values (10000000000000000000000000000000000000000000000000);
insert into t values (12345678901234567890123456789012345678901234567890);
select * from t;
X
10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
12,345,678,901,234,567,890,123,456,789,012,345,678,900,000,000,000
If you need more precision than this, read:
https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:9530645800346731533