If ten parties participate in multi-party calculations, but only calculate the result of multiplying the ciphertext of the third party and the fourth party, does decryption only require the third party and the fourth party to provide the secret key? If not, what do I need to prepare to decrypt?
I’d think you need as many as you would as if all parties had participated in the computation. Having said that, I’m not an expert
How many parties have a secret key share depends on how many parties need to participate in the distributed decryption. If only 3rd and 4th parties should be involved in decryption and other parties simply encrypt using the public joint key (in threshold FHE), then only these two parties need to have secret shares.