Generate Password for scratch org admin user

 By default, every new scratch orgs have one admin user with no password. Use the generate password CLI command to generate or change a password for any scratch org user.

Generate a password for scratch org admin user

sf org generate password

Generate a password for the specified users

If you want to generate password for other user in your scratch org, add Comma-separated list of usernames or aliases to assign the password to generate. 

sf org generate password -b "user1@my.org,user2@my.org"

Generate a password with number of characters

we can also generate password with specific number of characters and what characters to include in your password.

sf org generate password -l 8

Flags:

• -b/--on-behalf-of : Comma-separated list of usernames or aliases to assign the password to.
• -l/--length : Number of characters in the generated password; valid values are between 8 and 100.

Source:
https://developer.salesforce.com/docs/atlas.en-us.sfdx_cli_reference.meta/sfdx_cli_reference/cli_reference_org_commands_unified.htm#cli_reference_org_generate_password_unified