Does your mail provider automatically remove Trash messages older than, say, 30 days? With IMAP accounts, a folder is resynced when it's selected, so the actual number of messages is immediately updated.

Note that i can see but thanks for the suggestion

Many servers do auto remove emails that have been in the Trash for longer than a specific period. Servers assume that if you delete emails then it is because you do not want those emails. They are put in the Trash so you have the opportuniity to rescue them if deleted in error.

examples. Gmail auto empties the Trash every 30 days approx. Yahoo auto empty Trash every 7 days https://help.yahoo.com/kb/trash-spam-folders-regularly-emptied-sln3518.html GMX default setting keeps deleted emails in Trash for only one day, but you can customise this. https://support.gmx.com/email/organizing-and-searching/trash.html

what server are you using?

It is not advisable to delete emails you want to keep. It is not advisable to store emails you want to keep, in Trash.

Assuming you are using imap..

As sfhowes says, if the number changes when imap trash folder synchronises, then it is updating the imap account in Thunderbird to show what is on the server.

'Don't delete any messages' is a setting to prevent auto deletion of items you have not manually deleted. So emails you have manually selected to be deleted do not count.

'Don't delete any messages' setting can be set in 'Account settings' > 'Synchronisation & Storage' and used to apply to all folders in the account. However, that setting can be manually adjusted on a per folder basis via the folder's 'Properties'.

Do you use any other means of reading emails, eg: via phone or webmail or another computer using imap? If you access the server using other methods phones or computers, and you delete off server, then it will also be deleted in the Thunderbird imap account because it can only show what is on the server.