Suggestion - 𝑛 instances of each pony

[LoganDark]

You are able to choose 0 or 1, so why not add an additional option below it:

[text box] of each pony

Something like that. (Where [text box] is an input field accepting an integer)

[hidefromkgb]

Got a better solution; see the picture. «Group selection» area is a spin control attached to a button, on clicking which all selected ponies get N additional instances. N is taken from a spin control and may be negative.

[LoganDark]

@hidefromkgb Good idea