- a transaction with more than 40 coupons;
- a transaction with more than $50 worth of coupons;
- any coupon with a value above $20.
The Walmart coupon policy also specifically states that there's no maximum amount of coupons used per transaction -- so you should no longer need to split up your transactions, unless there's a limit placed on the coupon itself. For example, all P&G coupons specifically state that you can only use 4 coupons per identical item per transaction.
In order to cut down on frustration at checkout, I recommend finding your Walmart CSM (customer service manager) before getting in line, to let them know you'll be needing their assistance at the end of your large couponing transaction. It's also a good opportunity to get your CSM's advice on which cashier to use -- preferably someone who won't be freaked out by the sight of a coupon binder.