pg_xlog question

Questions on pg_xlog:

1.  what parameter tells postgress to always keep 7 files in the pg_xlog
directory? (see below) --this is test database we
are learning on. (and it seems everytime that I look at this directory there
are always 7 entries there)

2.  What happens when the files increment and can no longer increment to a
higher number (the file name)?

3.  Is there a way to "reset" the number so that this won't happen?


total 114800
-rw-------  1 pgsql  pgsql  16777216 May  6 19:07 00000009000000CC
-rw-------  1 pgsql  pgsql  16777216 May  6 19:08 00000009000000CD
-rw-------  1 pgsql  pgsql  16777216 May  6 19:09 00000009000000CE
-rw-------  1 pgsql  pgsql  16777216 May  7 08:23 00000009000000CF
-rw-------  1 pgsql  pgsql  16777216 May  7 08:25 00000009000000D0
-rw-------  1 pgsql  pgsql  16777216 May 22 10:32 00000009000000D1
-rw-------  1 pgsql  pgsql  16777216 Jul  9 16:23 00000009000000CB
$ pwd
/usr/local/pgsql/data/pg_xlog
$

Thank you
Pam Wampler