"We only have one pair of shoes."
Translation:Níl ach péire bróg amháin againn.
The thing to remember is that the "one" in "we have a new one" isn't actually a number, it's a pronoun (standing in for the thing that you have). That's why you can say "I have 3 new ones" - "three" is being used as a number, and so can be represented by the numeral "3", but "ones" isn't, it's just shorthand for "pencils" or "batteries" or "books", or whatever it is that you have.
So, when we translate tá ceann amháin agam as "I have one", amháin is the number 1, and ceann is an unspoken "thing" - you could just say tá ceann agam.
"he told us a good one" - d'inis sé ceann maith dúinn
"that's the sixth (one) in the series" - sin an séú ceann sa tsraith
"it's almost impossible to get one" - tá sé beagnach dodhéanta ceann a fháil
In this exercise, "We only have one pair of shoes." "one" is a number - you could write "We only have 1 pair of shoes", or you could use any other number, - two three, four etc. So you translate it as amháin, not ceann. And you have also explicitly mentioned what you have (péire bróg), so you don't need a pronoun (ceann) to stand in for it, as it does in the examples above, where ceann is standing in for joke, book and ticket for the Late Late Toy Show.