How many signatures are required to start the formal decision making stage of a direct democracy mechanism?

Constitution

http://www.tribunalconstitucional.es/es/constitucion/Paginas/ConstitucionIngles.aspx

Article 87

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3. An organic law shall establish the manner in which popular initiative inconnection with the submitting of non-governmental bills shall be regulated, aswell as the requirements therefore. In any case, no fewer than 500,000authenticated signatures shall be required. This initiative may not touch onmatters concerning organic law, taxation, international affairs or theprerogative of granting pardons.