With e-commerce websites in some cases you must limit quantity of products the user can get, but some don't have any specific rule for that. Criteria I can think you should account for is:
How many products you have on stock
How many products can you deliver at once
Limitations of payment processor
Would anyone actually want a huge amount that would possibly break the layout? Or should we account for nasty people like me that likes to break layouts:
As most of those are too time consuming to figure out, is there an arbitrary number that would work in most cases? Maybe a limit on the number of characters like 3 on the below example?