Here is a link to the SCOTUS docket where you can read all the filings of the parties involved. This is updated as more responses come in. The Defendants have until 3pm today to submit.
https://www.supremecourt.gov/docket/docketfiles/html/public/22o155.html

***Update*** Missouri, Arkansas, Louisiana, Mississippi, South Carolina and Utah have now also filed to "Intervein" as additional Plaintiffs. Yesterday, these states and others were part of filing an Amicus Brief. Now, they also want to have skin in the game.
https://www.supremecourt.gov/DocketPDF/22/22O155/163215/20201209144840609_2020-12-09%20-%20Texas%20v.%20Pennsylvania%20-%20Amicus%20Brief%20of%20Missouri%20et%20al.%20-%20Final%20with%20Tables.pdf
***Update***
The state of Ohio files an Amicus brief taking neither side, but asks SCOTUS to issue a decision on the meaning
of the Electors Clause at the earliest available opportunity.
https://www.supremecourt.gov/DocketPDF/22/22O155/163346/20201210125506698_TX%20v%20PA%20-%20Amicus.PDF
https://www.supremecourt.gov/docket/docketfiles/html/public/22o155.html

***Update*** Missouri, Arkansas, Louisiana, Mississippi, South Carolina and Utah have now also filed to "Intervein" as additional Plaintiffs. Yesterday, these states and others were part of filing an Amicus Brief. Now, they also want to have skin in the game.
https://www.supremecourt.gov/DocketPDF/22/22O155/163215/20201209144840609_2020-12-09%20-%20Texas%20v.%20Pennsylvania%20-%20Amicus%20Brief%20of%20Missouri%20et%20al.%20-%20Final%20with%20Tables.pdf
***Update***
The state of Ohio files an Amicus brief taking neither side, but asks SCOTUS to issue a decision on the meaning
of the Electors Clause at the earliest available opportunity.
https://www.supremecourt.gov/DocketPDF/22/22O155/163346/20201210125506698_TX%20v%20PA%20-%20Amicus.PDF