must v. may wikidiff

As verbs the difference between must and may is that must is to do with certainty; (indicates that the speaker is certain that the subject will have executed the predicate)

and

may 1. To be allowed or permitted to: 2. Used to express possibility or probability:

must v. may

must TFD:

• 1. To be obliged or required by morality, law, or custom:
  2. To be compelled, as by a physical necessity or requirement:
  3. Used to express a command or admonition:
  4. To be determined to; have as a fixed resolve; inevitability or certainty; logical probability or presumptive certainty

and

may TFD

• 1. To be allowed or permitted to:
  2. Used to express possibility or probability:
  3. Used to express a desire or fervent wish:
  4. Used to express contingency, purpose,

Some overlap, some confusion, but in a contract: " you may only have one account" is not a suggestion. If you choose to enter into this contract you will (may) have one account.