Output amplitute from an NCO

I will assume now your DAC has no sinc correction. You will get amplitude droop(due to hold device similar to low pass average filter for the duration of one sample). This droop goes towards zero at Fs and its multiples(after successive lobes). The remedy is to add inverse since filter somewhere e.g. in the fpga or in the dac(some dac have it) or in the analogue domain.