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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:

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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?

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Communicate why there is a limit

Item limits can vary greatly depending on the situation so there isn't a single value that can be applied across the board.

It is frustrating anytime an interface prevents logically valid input without explaining why. In an e-commerce situation the best way to communicate reality to a user is to simply tell them your available stock on hand. People understand that there isn't an unlimited supply of goods and services so letting them know how many items are available to purchase is great.

Another approach is to let users know you want to service as many people as possible so there is a 5 or 10 item maximum purchase per household.

An added benefit to telling a user what's available

There is good evidence to suggest that showing a user the availability of stock on hand increases sales. It's the simple economics of supply and demand.

scarcity

0

I can see cases for the first three of your criteria and one other that you haven't mentioned: enforced rationing - The retailer needs to limit the amount of a particular item to each consumer for whatever reason.

Your fourth criterion, however, makes no sense: the consumer will request the amount that they want.

Also, the examples you show involve figures that would probably buy the entire western world. Looking specifically at the Google example, I counted enough digits to run into trillions of pounds long before the number comes out of the grey box - If someone is willing to give you that amount of money do you really care if they break the layout?

The rule of thumb that I use is to calculate five time the price of the most expensive product in the catalogue and then add another digit.

  • Your answer may be no, we shouldn't account for the rare occasion of someone wanting to buy thousands of products or layout breaks but the question still makes sense as there is the possibility to input unlimited numbers. However thank you for your rule of thumb. – Taly Emmanuela Jun 11 '15 at 13:18

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