@eduardosilvin You'll need to switch to the Classic Designer to follow along. Click the three dots (top right of the screen) and select Switch to classic designer.

Get Emails (V3) Limitations
It's important to note that the Get emails (V3) action has a limit of 25 emails that it'll return. The default is 10. I'll explain how to bypass this limit below.

Get Email Count
To return a count of items you need to use an expression. Add a Compose action to store the expression. Click on the Expression tab and insert the length() function.
length()

With your cursor in between the brackets, select the Dynamic Content tab. Insert the value dynamic content from the Get Emails (V3) action.

Run a test. The number of unread emails will be stored in the Compose action.

Send an HTTP Request (Outlook)
You can bypass the 25 email limit by using a Send an HTTP Request action instead. Insert the text below into the URI field. Customize the top count number to suit your requirements.
https://graph.microsoft.com/v1.0/me/messages?$filter=isRead eq false&top=100

Run a test. Copy the outputs to your clipboard. Tip: Place your cursor in the outputs and press CTRL + A or CMD + A to select all.

Add a Parse JSON action. In the Content field insert the body dynamic content from the Send an HTTP Request action.
Press the Generate from Sample button.

Paste the content from your clipboard and press Done.

In a Compose action use the length() function (refer to instructions above) and insert the value dynamic content from the Parse JSON action.

Run a test. Verify the outputs.

Add Count to Teams Post
In your Post message in a chat or channel action, insert the Outputs from the Compose action that is storing the number of unread emails into the Message box.

For clarification:
If you are using the Send an HTTP request action, your flow should look like this

If you are not using the Send an HTTP request action, your flow will look like this

Hope this helps!
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