iPhone Mail app shows incorrect junk email count

if it was me then I would likely try to force close the mail app

Close an app on your iPhone or iPod touch - Apple Support

and try to start it up again

otherwise I would restart the iphone

Force restart iPhone - Apple Support

"iPhone Mail app shows incorrect junk email count: Hi my iPhone iOS email account shows I have 21828 junk emails when in fact the folder is empty. It’s been like this for some time. How do I reset this to 0 where it should be?"

-------

Troubleshooting Apple Mail App on iPhone:

New iCloud Session:

Log out of your Apple Account on your iPhone, and then log back it, toggling on Mail in iCloud Settings. It ought to reset.

Your emails are not downloaded yet. (Unread mails are on the email client-server)

iCloud - Both Fetch and Push Mail option is available

1. Push Mail: Whenever possible the email client will push emails to all your devices.
2. Fetch Mail: You need to set How you want it to be fetched, Automatically, Manually, Hourly, Every 30 mins, every 15 mins
   1. Automatically: will fetch when the iPhone is powered and on WiFi
   2. Manually: with fetch when you open the email, open Inbox, and swipe down
   3. You get an email notification as soon as the client-server receives an email and your device is connected to the internet, irrespective of whether the mail is in your device's Inbox or not

Which email account are you referring to is not clear from your post. Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other

For email, this is called Push / Fetch Settings. You may know that there are two settings (Fetch and Push Mails). What have you set? Watch the GIF below

Google, AOL, Yahoo - Only the Fetch mail Option is available

iCloud, Exchange, Outlook - Both Fetch and Push Mail option is available

1. Push Mail: Whenever possible the email client will push emails to all your devices.
2. Fetch Mail: You need to set How you want it to be fetched, Automatically, Manually, Hourly, Every 30 mins, every 15 mins
   1. Automatically: will fetch when the iPhone is powered and on WiFi
   2. Manually: with fetch when you open the email, open Inbox, and swipe down
   3. You get an email notification as soon as the client-server receives an email and your device is connected to the internet, irrespective of whether the mail is in your device's Inbox or not

The pic below is self-explanatory

is this an iCloud account or is it a pop/imap account?