cua-enable-register-prefix

cua-enable-register-prefix is a variable defined in `cua-base.el'.
Its value is
not-ctrl-u


Documentation:
If non-nil, registers are supported via numeric prefix arg.
If the value is t, any numeric prefix arg in the range 0 to 9 will be
interpreted as a register number.
If the value is `not-ctrl-u', using C-u to enter a numeric prefix is not
interpreted as a register number.
If the value is `ctrl-u-only', only numeric prefix entered with C-u is
interpreted as a register number.

You can customize this variable.