I created a table for testing purpose
create table xx (a number)

inserting 38 digit number
insert into xx values(11111111111111111111111111111111111111)

inserting 39 digit number
insert into xx values(111111111111111111111111111111111111111)

inserting 126 digit number
insert into xx values(1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111)

if i try to insert 127 digit number then it is showing message ora - 01426 numeric overflow.

Precision for oracle numeric data is 38 only. How it is accepting a numeric value upto 126 digits?

For Example ,if precision is defined
create table xx (a number(38))
----
inserting 39 digit number
insert into xx values(111111111111111111111111111111111111111)
then it will return error ora-01438
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if not defined then
the precision is the maximum number of significant decimal digits i.e 126 binary digits

Whether numeric datatype is fixed length datatype or variable length datatype? i.e. Whether if takes fixed length irrespective of value we stored just like char datatype or it takes space depending on the value we stored?

I found answer. Numbers in an oracle database are always stored as variable length data.