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

Serbia, Constitution 2006, accessed 20 December 2021

Article 107: Right to propose laws

A right to propose laws, other regulations and general acts shall belong to every deputy, the Government, assemblies of autonomous provinces or at least 30,000 voters.

Article 203: Proposal to amend the Constitution and adoption of the amendment to the Constitution

A proposal to amend the Constitution may be submitted by at least one third of the total number of deputies, the President of the Republic, the Government and at least 150,000 voters.

The National Assembly shall decide on amending the Constitution.

A proposal to amend the Constitution shall be adopted by a two-third majority of the total number of deputies. […]

Serbia, Law on the Referendum and the People’s Initiative (December 2021) [in Serbian], accessed 20 December 2021

Article 46. The National Assembly shall call a referendum on the territory of the Republic of Serbia when the citizens should decide:

1) on issues determined by the Constitution;

2) on issues falling within the competence of the National Assembly upon the request of the majority of all Members of Parliament;

3) on issues falling within the competence of the National Assembly upon the request of at least 100,000 voters.

Article 50. When at least 100,000 voters have submitted a request for calling a referendum, the Speaker of the National Assembly shall proceed such request to the Members of Parliament, the Government and the committee of the National Assembly competent for constitutional issues (hereinafter: competent committee of the National Assembly).

For optional referendum (also citizens' initiative) - 100,000 voters; for mandatory referendum (Constitutional amendment) - 150,000 voters; for agenda initiative - 30,000 voters' signatures are needed.