How many official "Republics" are there in the Star Wars universe?

I was wondering how many official "Republics" there are in the Star Wars universe?

One, two, three? I know about the Old Republic and the New Republic but I was wondering if there are any more.

Can the answerer please use the Star Wars movies or Thrawn trilogy (I am reading the first book now) as reference because only some of my Star Wars knowledge is on the Expanded Universe.

Note: to limit confusion in answers please show your estimated time periods for each republic.

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