I just wanna ask that after rooting Nexus 5 will I be receiving OTA updates by Google ? Or I'll have to install a custom ROM ?
Yes, OTA updates will still be suggested to your device when you're rooted. Under most circumstances, they will also be applied. However, be advised that if you performed any root-required tasks that altered system files, such as editing build.prop, freezing apps, removing apps, etc, then the OTA may fail.
When the OTA does come, if you apply it OTA rather than flashing a pre-rooted image, you will most likely lose root, and Towelroot will no longer root the device.