Answer:
Nick advises Gatsby "I wouldn't ask too much of her" at the end of Chapter 6 because he understands that Gatsby is placing too much emphasis on his relationship with Daisy and is setting himself up for disappointment.
Gatsby has built up Daisy in his mind to be an idealized version of herself, and he believes that if he can just win her back, everything will be perfect. However, Nick recognizes that Daisy is not the perfect woman that Gatsby has made her out to be, and that she is flawed just like everyone else. Nick also understands that Daisy is torn between her feelings for Gatsby and her loyalty to her husband, Tom.
By telling Gatsby not to ask too much of Daisy, Nick is warning him not to put all of his hopes and dreams into one person, especially someone who may not be able to fulfill them. Nick wants Gatsby to be realistic about his expectations and to understand that there are limits to what Daisy can offer him.