PHP, 16 bytes

$argn/sqrt(2)|0;

Try it online! (with highest value supported on TIO)

Uses @Niphram's truncate method (which in PHP also has the ability to convert the float to an int)

I know it's trendy to say PHP is to be hated, but I kinda came to like its oddities, and it gives me a chance to add an original answer

EDIT: saved 4 bytes using <?= php tag (no need to echo)

PHP, 17 bytes

<?=$argn/2**.5|0;

Try it online!

Uses @Niphram's truncate method (which in PHP also has the ability to convert the float to an int)

I know it's trendy to say PHP is to be hated, but I kinda came to like its oddities, and it gives me a chance to add an original answer

EDIT: saved 4 bytes using <?= php tag (no need to echo)

EDIT2: basically it's just a port of @Niphram's answer now

PHP, 20 bytes

echo$argn/sqrt(2)|0;

Try it online! (with highest value supported on TIO)

Uses @Niphram's truncate method (which in PHP also has the ability to convert the float to an int)

I know it's trendy to say PHP is to be hated, but I kinda came to like its oddities, and it gives me a chance to add an original answer

PHP, 16 bytes

$argn/sqrt(2)|0;

Try it online! (with highest value supported on TIO)

Uses @Niphram's truncate method (which in PHP also has the ability to convert the float to an int)

I know it's trendy to say PHP is to be hated, but I kinda came to like its oddities, and it gives me a chance to add an original answer

EDIT: saved 4 bytes using <?= php tag (no need to echo)