Chase:

• 5/24 Rule: If you have more than five new credit cards in the past 24 months, from any issuer, you’ll automatically get rejected for certain Chase cards.
• 2/30 Rule: You can only have two applications every 30 days or else you’ll automatically be rejected.
• One Sapphire Rule: You cannot apply and get approved for a Sapphire card if you currently have an open one.
• Chase Sapphire 48-Month Rule: You can only receive an intro bonus to a Chase Sapphire card once every 48 months (4 years). The rule begins when you get the bonus, not the card.

Citi:

• 48-Month Rule (AA Cards): Citi’s 48-month rule restricts certain credit card welcome bonuses to once every 48 months. So if you apply for one of the affected credit cards now and decide to cancel the card in two years, you will not be eligible to reapply and receive the bonus again until 48 months after your first application.
• 8/65 Rule: Citi's 8/65 rule states that applicants are ineligible to apply for more than one Citi credit card in eight days, and not more than two credit cards in 65 days.
• 24-Month Rule: You typically can’t get a welcome bonus on a particular card if you’ve received a welcome bonus or closed that card (or a card in that family) in the past 24 months.

Amex:

• Once In A Lifetime: If you've ever had a certain Amex product past or present, regardless of if you received a signup bonus, you will not be eligible for the signup bonus again.
• 5 Card Limit: Some data points suggest that you can only have 5 American Express credit cards at one time, regardless if it’s a personal or business card. Your mileage may vary.
• 1 In 5 Rule: You can only get approved for one credit card every five days.
• 2 In 90 Rule: You can only get approved for two credit cards every 90 days. This means that if you apply for a third card within the 90-day window, you'll automatically be rejected.