suffix, counter
counter for bags of something
Attached to numbers to count bagged items. Used with native Japanese numbers (ひと, ふた, み, etc.) or Sino-Japanese numbers (いち, に, さん, etc.), though native numbers are more common in everyday speech.
お菓子を二袋買った。
I bought two bags of sweets.
このお茶は一袋に何グラム入っていますか。
How many grams are in one bag of this tea?
本 is a general counter for long, cylindrical objects, not for bagged items.
From the noun 袋 (ふくろ, 'bag'), used as a suffix/counter with the reading たい, likely derived from a historical compound or on-reading usage.