Most of the info is in the Holo wikia page.
She was able to travel with Lawrence because she can "jump" between wheat, and she left the wheat that was in the village's fields and to the wheat that Lawrence bought, giving the reason that she longed for her snowy homeland and eventually contracts Lawrence to take her there.
Holo is a wolf harvest deity that was bound to the town of Pasloe, ensuring a bountiful harvest for the residents. As the villagers became more self-reliant with time, however, Holo was neglected and, feeling unneeded, skipped town with the traveling merchant, named Lawrence, by 'jumping' from the villagers' wheat to Lawrence's wheat in his wagon. She longs to return to Yoitsu, her snowy homeland, and makes a contract with Lawrence to accompany her there.
Since Lawrence makes a living being a trader, he can't keep carrying wheat around so Holo keeps some in a pouch. But eating wheat isn't the only way she can transform:
Holo has the power to transform into a massive wolf (bigger than an elephant) in this form she uses this to move at high speeds much faster than another known creature. She also uses her huge jaws in order to defend herself and Lawrence (as well as scare the crap out of her enemies). To activate this form she must either consume wheat or human blood.
As far as "dwelling" in the wheat, it also says:
Holo has lived for many centuries and is not visibly aged past 17 years old (at least in her human form). She credits her wisdom from the experiences her long life has given her. However she is not immortal, she explains that if the wheat she dwells in is destroyed, she would disappear.
So it's unlikely she'd eat all the wheat knowing she'd disappear if there was no more wheat to "dwell" in. I don't recall much explanation at all about the wheat aside from the initial bullet points, but I imagine it wouldn't be out of the question to occasionally replenish some of the wheat in her pouch through their travels.