You don't have to worry about the locks because Postgres uses MVCC (Multi Version Concurrency Control). You can find out more about MVCC in here:

• http://en.wikipedia.org/wiki/Multiversion_concurrency_control
• http://www.postgresql.org/docs/8.3/static/mvcc.html

MVCC basically means that more copies of a row coexist in one table thus eliminating the need for locks. The only thing you need to worry about that your counter table will occupy more and more disk space even if it contains only one row. You can free space by doing VACUUM FULL but a full vacuum requires exclusive lock.

If you are sure the estimated value is good enough you can get it this way:

select reltuples from pg_class where relname='some_table'

With a lot of patience and a lot of googling.

The page http://www.postgresql.org/docs/8.3/interactive/catalog-pg-class.html describes pg_class but I tell you my secret how did I find out about it's existence:
In the postgres config file you change log_statement to all reload the config file then you start pgadmin and watch what it does.
Well, it does very complicated queries but with a little experimenting you can learn a lot, for example this is how you get the list of tables in a database:

SELECT table_name FROM information_schema.tables WHERE table_schema = 'public'

Just don't forget to turn off statement logging when you are done experimenting because your log will grow very fast.