It depends a lot on the format, skill level, and your goals.
Cash games: around 4–8 tables to maintain a good quality of decision-making.
MTTs (tournaments): usually 4–8 tables, since each table requires more attention and the situations are constantly changing.
Sit & Go / fast-paced formats: 8–16+ tables, especially when using software and highly automated routines.
High stakes: often fewer tables, sometimes just 1–4, because each decision has much more financial impact.
Micro/low stakes: 12–20+ tables, sacrificing some decision-making quality in exchange for more volume per hour.