The Transfer on Death account type does not accommodate discrete or fixed dollar distributions, but rather distributions that are percentage based (as the value of the account cannot be known for certain in advance of the account holder's death). Accordingly, you may designate up to 6 primary beneficiaries as long as the percentage distribution totals 100%.
In addition, you may designate one or more optional contingent or secondary beneficiaries in the event a primary predeceases the account owner or allocate that deceased primary beneficiary's share to the remaining primary beneficiaries or to your estate.
Please see the Interactive Brokers Knowledge Base for additional information concerning the Transfer on Death account type.