This specific count (6000) is significant because it avoids the bloat of full 10,000+ ROM sets (which include a ton of mahjong, fruit slot machines, and prototype games nobody plays). The 6000 range is considered the "sweet spot" for a home arcade cabinet.
MAME Plus + 6000 ROMs + Extras Deluxe pack is a well-known, community-curated collection designed to provide a "plug-and-play" arcade experience. Typically found on file-sharing sites like Chomikuj.pl mame plus 6000 roms extras deluxe link