It's not set in stone. Generally, agents and publishers look for manuscripts that are neither too short to fully develop the story and characters nor too long to be commercially viable. For example, a fantasy novel might need to be longer to build a complex world, while a YA novel might be better around 50,000 to 70,000 words. It all depends on multiple factors.
The preferred length can differ a lot. For a debut novelist in certain genres like romance or mystery, a manuscript around 80,000 to 90,000 words might be ideal. However, for literary fiction, a lengthier manuscript could be acceptable if the story justifies it. Publishers also consider market trends and reader expectations when it comes to manuscript length.
It varies. Some agents and publishers prefer manuscripts in the range of 70,000 to 100,000 words for adult fiction, but it really depends on the genre and the specific requirements of each.