Creating Masked text fields with OCMaskedTextField is extremely easy and this code example from the readme shows how to set up a text field with masking for number entry:

OCMaskedTextFieldView *textField = 
    [[OCMaskedTextFieldView alloc] initWithFrame:CGRectMake(20, 94, 280, 33)
                                   andMask:@"TR - #### #### #### #### #### ####"
                                   showMask:YES];

Here’s an image showing several masked text fields from the readme:

You can find OCMaskedTextField on Github here.

A handy library to have if you’re looking to create some masked text/number entry fields.