How significant is it that 13 countries have joined or declared their intention to join South Africas genocide case against Israel ?

Indeed, it is very significant. Spain has announced its intention to participate in the legal proceedings initiated by South Africa at the International Court of Justice (ICJ). The complaint alleges that Israel has breached its commitments under the Genocide Convention during its military operations in the Gaza Strip.
Spanish Foreign Minister Jose Manuel Albares declared:

“We made this decision in light of the continuation of the military operation in Gaza.”

In December, South Africa lodged a case against Israel, alleging that it has engaged in acts of genocide against Palestinians in Gaza. According to health officials in the besieged and bombarded zone, the death toll from Israel's conflict, which started in October, has exceeded 40,000.
South Africa's case to the United Nations court in The Hague asserts that Israel has contravened the 1948 Genocide Convention, a treaty created in response to the Holocaust, which mandates all nations to take measures to prevent the repetition of such atrocities. These kind of litigation can require several years to reach a resolution.
All states that ratified the 1948 Genocide Convention are legally obligated to refrain from engaging in acts of genocide and to take measures to prevent and punish such acts. The treaty provides a precise definition of genocide as:

“Acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group”.

States have the right to intervene in the case before the International Court of Justice (ICJ) if they have a legal interest that could be impacted by the court's ruling. They can file a request to the court to be permitted to intervene, which the court can then decide upon.
The countries that have either joined or expressed their intention to join South Africa's genocide case against Israel are listed below:
  1. South Africa: Filed the case on December 29.
  2. Nicaragua: Applied to join on February 8.
  3. Belgium: Declared intention to join on March 11.
  4. Colombia: Applied to join on April 5.
  5. Turkey: Declared intention to join on May 1.
  6. Libya: Applied to join on May 10.
  7. Egypt: Declared intention to join on May 12.
  8. Maldives: Declared intention to join on May 13.
  9. Mexico: Applied to join on May 24.
  10. Ireland: Declared intention to join on May 28.
  11. Chile: Declared intention to join on June 2.
  12. Palestine: Applied to join on June 3.
  13. Spain: Declared intention to join on June 6.
Many other nations and organizations have expressed support and welcomed South Africa's case amid a global chorus for a ceasefire in Gaza.