Actually, Microsoft Outlook provides the Messages in this Conversation feature to help you quickly search for all emails by conversation.

In the Mail view, right click the specified email which belongs to the conversation you will search by, and select Find Related > Messages in this Conversation from the context menu. See screenshot:

Now the search criteria [Conversation]:="test search emails by conversation" (test search emails by conversation is the specified email’s subject without prefix) is added into the Instant Search box automatically, and all emails in the specified conversation are found out and listed in the mail list. See screenshot:

Find related messages in Outlook

Count the number of emails by subject in Outlook