How to SELECT number from table and use in calculation . Unsupported operand types

$mpcountrys is going to be an array or an object. In your case, an object, which is the default.

One of four pre-defined constants. Defaults to OBJECT. See SELECT a
Row and its examples for more information.

https://codex.wordpress.org/Class_Reference/wpdb#SELECT_Generic_Results

It looks to me like you probably want get_var() instead.

And I would highly recommend a bit of sanitaziation/validation of your user supplied values before trying to use them.

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