Are you looking to like MAMEdev or a front-end like LaunchBox with your ROMs?

Each game and every "clone" (variation) contains all the necessary data to run. This includes the parent ROM data and any required BIOS files.

A full non-merged MAME 0.260 set (around 8,000+ working games) can be ~300-400 GB , compared to ~180 GB for a split set and ~120 GB for a merged set.

Unlike other formats that rely on "parent" files to save space, a non-merged set ensures that if you want to play a specific version of a game (like a Japanese release or a "bootleg" clone), you only need that one file to run it. Core ROM Set Types