Analog CCTV camera - the audio input is directly fed into the DVR. It means, between the DVR and the microphone (audio input device), it is the analog signal transmission. The digitalisation of the audio signal is done at the recorder level.
The limitation of how many microphones can be installed of this solution would be the number of inputs allowed by the DVR.
IP CCTV camera - the audio input is fed into the IP Camera. Between IP camera and the microphone, it’s the analog signal transmission. However, digitalisation of the audio signal is done at the IP camera level. And it will be digital signal to transmitted to the NVR.
The limitation of how many microphones can be installed, for this solution, would be purely depend on the number of video channel of the NVR, and whether the IP camera comes with the audio input. Say, a 8 channel NVR, with all the 8 audio-input-available IP cameras installed, can accommodate up to 8 microphones.