How can I see the sfdx cli version Number?

Sfdx CLI Version Information under Troubleshoot Salesforce DX covers this
sfdx plugins --core // Version of the CLI and all installed plug-ins sfdx --version // CLI version
The rest of the CLI commands are noted in Salesforce CLI Command Reference as well as easily shown in the CLI with the following
sfdx commands

And it's always possible to add --help within a given command to understand any specific format/info about it. You can even just do:

  sfdx --help

which outputs the following (also displays version).

Salesforce CLI VERSION sfdx-cli/7.110.0 darwin-x64 node-v14.17.6 USAGE $ sfdx [COMMAND] TOPICS alias manage username aliases auth authorise an org for use with the Salesforce CLI config configure the Salesforce CLI force tools for the Salesforce developer plugins add/remove/create CLI plug-ins COMMANDS autocomplete display autocomplete installation instructions commands list all the commands help display help for sfdx plugins list installed plugins update update the sfdx CLI which show which plugin a command is in