I've just been trying to gather some seeds for planting. Trying being the key word.
I find that often when I pop a number into the input box thing it will double the number. Like say I type 3, half the time it will jump to 33 (and if my total of an item is less than that it'll jump to the max number of the item). It means a lot of the time I end up having to retype the amount, but that isn't much of a problem.
The real problem crops up if you're trying to do things fast or having to deal with massive lag as I am today. Because that number doubling? It happens right at the last second, just before you press enter. I was lucky in that I've only lost about 6 cabbages in this lot of seed gathering, but I've lost a lot more previously because of this. And it isn't my keyboard making it double, because I can type things in anywhere else like when making recipes etc, and I've never had the numbers double.
Of course a nice little 'feed 3 of this thing' option that was always available for piggies (regardless of the amount of an item you have) would be the most preferable solution. Or even a 'Seedification' option on the piggies that feeds them 3 of any crop you select. That way those wanting to feed piggies to keep them alive can, and those wanting seeds can have their own option too (and don't have to worry about losing large numbers of their crops to piggys, I think it would also speed up the process a bit as you wouldn't have to go piggy-feed-item-amount, instead you would go piggy-seedification-item.
Seed gathering is a tedious enough process as is without having to worry about numbers changing like this.