Deductible contributions to a Michigan 529 plan are limited to $10,000 for married couples filing jointly or $5,000 for individuals filing single per year. This is total, not per account.
Enter only $10,000 or $5,000 even if you contributed more. TurboTax will give you an error if you exceed those amounts, as those are the deductible limited.
Related Resource:
About 529 Plans: Is there a Michigan income tax deduction?