。
The forms are numbered sequentially.
Literal
Forms [topic-は] in-order number [object-を] is-attached-[てある].
~てある marks deliberate resultant state — someone has stamped numbers on them and they remain so. 打つ ('to hit / strike') is used for 'printing / stamping' numbers or marks. 順に ('in order / sequentially') is an adverbial.