Steps to reproduce:
1. Add argument Global:Null (it might work with other arguments as well, but this seemed the least intrusive)
2. Go to ‘Specify validation criteria’ select ‘php code’ and add this:
You don’t need to change anything else in the argument – however in the ‘More’ section it might be useful for future reference to change the argument name from ‘Global: Null’ to something like ‘Number of items’
This is especially useful for views you would like to include in nodes using the Viewfield or Insert View module, where you would like editors to be able to add views with the number of items they want without having access to the view structure.