What final-format do you want? What are you going to do with the final file? Make a CD? Play it on your phone? Play it in your car?
WAV is the most common lossless format. FLAC is also lossless but the files are almost half the size and “tagging” is better supported than WAV, but it’s not as widely supported as some other formats.
After recording your vocals export to WAV at 16 or 24 bits, and you can optionally save an Audacity project (which is also lossless).
When you export the vocal + backing mix, export (temporarily) as floating-point WAV (which can go over 0dB). …Mixing is done by summation so the mixed levels can go over 0dB and you can end-up with clipping (distortion) if you export as a regular (integer) WAV.
Re-import the floating-point WAV and run the Amplify or Normalize effect to bring the peaks down to 0dB (if necessary) and then export to whatever final format you want.
MP3 and AAC (MP4/M4A) are the most popular lossy formats and with high-quality settings they can often sound identical to the lossless original. But if you want a lossy compressed file, compress ONCE as the last step. Don’t do production with lossy formats (unless you are stuck with a lossy original and you have no choice). If you open an MP3 in an audio editor it gets decompressed. If you re-export as MP3 it goes through another generation of lossy compression and the “damage” accumulates.