That really depends on your body composition, sex, activity levels, and overall health.But to make things simpler, eating anywhere between 20 to 50 g of net carbs per day should put you in ketosis and keep you there .
There are other ways to get into and maintain ketosis other than just limiting carbs. Intermittent fasting and carb cycling in conjuction with vigorous exercising also works  when practiced alongside keto.
Being in optimal ketosis will give you the most benefits, but even limiting your carbs slightly or bordering on ketosis can help you. That's because the more carbs you eat, the less fat you'll burn and the greater your appetite will be, which makes weight loss harder.
Some ketoers a strict with their keto regimen while others allow some flexibility. It all really boils down to personal preferences as well as physiological limitations and differences.