Im trying to use xcrun altool to get my app notarised, what is --asc-provider parameter?
altool manual page
The manual for altool can be viewed by running the command man altool. The manual states:

--asc-provider provider_shortname
Required with --notarize-app and --notarization-history when a user account is associated with multiple providers.

Without your knowing, it is unlikely that your account is associated with multiple providers.
asc-provider
If you believe you need a provider, see Apple's Customizing the Notarization Workflow:

You can omit the asc-provider option for the notarization call if your credentials are associated with only a single team. Otherwise, use the option to indicate the provider short name for the appropriate team. Starting in Xcode 11, you can use altool to get a table of providers, provider short names, and team IDs associated with a given username and password using the list-providers option:
xcrun altool --list-providers -u &quot;AC_USERNAME&quot; -p &quot;@keychain:AC_PASSWORD&quot;