I have created a custom field - number field, what is the smart value for this, please?

The smart value would be {{issue.Worked Today}} or {{issue.customfield_xxxxx}}. You can get the custom field number if you click on the three dot menu and over over one of the options, the URL displayed will show the ID at the end.

Both ways send the email with blank in this specific field:

1.

This is the daily work log report.
{{#lookupIssues}}

* Issue Key: <b>{{key}}</b> (<a>{{url}}</a>)
* Summary: <a>{{Summary}}</a>
* Assignee: <c>{{assignee.displayName}}</c>
* Epic: <d>{{issue.epic.name}}</d>
* Priority: <e>{{issue.priority.name}}</e>
* Hours Worked Today: <f>{{issue.Worked Today}}</f>

{{/}}

2.This is the daily work log report.
{{#lookupIssues}}

* Issue Key: <b>{{key}}</b> (<a>{{url}}</a>)
* Summary: <a>{{Summary}}</a>
* Assignee: <c>{{assignee.displayName}}</c>
* Epic: <d>{{issue.epic.name}}</d>
* Priority: <e>{{issue.priority.name}}</e>
* Hours Worked Today: <f>{{issue.customfield_10063}}</f>

{{/}}