Searching for key word "exceeded" yields these results for myBB error messages.
"your upload number is exceeded" is not specific enough to narrow the problem down.
Upload/warnings.php: // Check we haven't exceeded the maximum number of warnings per day
Upload/inc/languages/english/admin/style_themes.lang.php:$l['error_uploadfailed_php1'] = "PHP returned: Uploaded file exceeded upload_max_filesize directive in php.ini. Please contact your forum administrator with this error.";
Upload/inc/languages/english/admin/style_themes.lang.php:$l['error_uploadfailed_php2'] = "The uploaded file exceeded the maximum file size specified.";
Upload/inc/languages/english/messages.lang.php:$l['error_maxposts'] = "I'm sorry, but your daily post limit has been exceeded. Please wait till tomorrow to post further or contact your administrator.<br /><br />The maximum amount of posts you may make in a day is {1}";
Upload/inc/languages/english/messages.lang.php:$l['error_uploadfailed_php1'] = "PHP returned: Uploaded file exceeded upload_max_filesize directive in php.ini. Please contact your forum administrator with this error.";
Upload/inc/languages/english/messages.lang.php:$l['error_uploadfailed_php2'] = "The uploaded file exceeded the maximum file size specified.";
Asking to be specific to rule out host issues on server. "Unlimited" never is.