You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Switzerland

Context:
Similarly, the federal constitutional initiative allows citizens to put a constitutional amendment to a national vote, if 100,000 voters sign the proposed amendment within 18 months.[note 8] Parliament can supplement the proposed amendment with a counter-proposal, and then voters must indicate a preference on the ballot in case both proposals are accepted. Constitutional amendments, whether introduced by initiative or in Parliament, must be accepted by a double majority of the national popular vote and the cantonal popular votes.[note 9]

Question:
How many voters are needed to put a constitutional amendment to a national vote?

100,000