Mangara, the Diplomat

• WOTC Official Reminder Textofficial rules
  Lifelink — Damage dealt by this creature also causes you to gain that much life.

• WWizards of the Coast2020-06-23
  Official Ruling
  You draw just one card, no matter how many creatures are attacking you and your planeswalkers beyond the second.
• WWizards of the Coast2020-06-23
  Official Ruling
  Players can cast spells and activate abilities after the triggered ability resolves but before the spell that caused it to trigger does.
• WWizards of the Coast2020-06-23
  Official Ruling
  If your opponent attacks you with one creature and your planeswalker with another, you draw a card.
• WWizards of the Coast2020-06-23
  Official Ruling
  An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
• WWizards of the Coast2020-06-23
  Official Ruling
  If some of the attacking creatures leave the battlefield while Mangara's triggered ability is on the stack, use the player or planeswalker they were attacking before they left to determine whether you draw a card. On the other hand, if they're removed from combat without leaving the battlefield, use their current information to determine that they're not attacking you or a planeswalker you control anymore.

Rulings via Scryfall