I checked online and they are suggesting to save your username pass in user global settings file, which I checked and Tortoise already saved them as plain text. But it's still asking me the same username and pass 50 different times when I want to update to a specific changelist.
There are two ways:
Save your username and password in mercurial.ini
:
[auth]
xxx.prefix = https://bitbucket.org/
xxx.username = USERNAME
xxx.password = PASSWORD
Use SSH keys instead and follow setup steps in this post: Set up SSH for Mercurial. This way doesn't need to store your password in plain text.