Yoreh Deah Siman 19 Seif 7

 

ז הָיוּ לְפָנָיו הַרְבֵּה לִשְׁחֹט, וּבֵרַךְ עַל הַשְּׁחִיטָה, וְאַחַר כָּךְ הֵבִיאוּ לוֹ עוֹד, אִם כְּשֶׁמְּבִיאִים לוֹ הָאַחֲרוֹנוֹת יֵשׁ עֲדַיִן לְפָנָיו מֵאוֹתָם שֶׁהָיוּ לְפָנָיו כְּשֶׁבֵּרַךְ, אֵין צָרִיךְ לַחֲזֹר וּלְבָרֵךְ; וְאִם לָאו, צָרִיךְ לְבָרֵךְ. וּלְכַתְּחִלָּה טוֹב לִזָּהֵר לִהְיוֹת דַּעְתּוֹ בִּשְׁעַת בְּרָכָה עַל כָּל מַה שֶּׁיָּבִיאוּ לוֹ.

Seif 7

If their were before him many (animals) to slaughter, and he blessed over the slaughter, and afterwards came more (animals), if when he brought the last ones, there are still before him those that he blessed, there is no need to repeat the blessing. And if not you need to re-bless.

And before the fact it is good to have in mind when reciting the blessing that is will count over all that is brought to him (even those not there yet).