The deadline for rolling over a code-M distribution is the due date of the tax return, including extensions, not the usual 60-day deadline to complete an indirect rollover.
There is no limit on the number of deposits to complete the rollover, only a limit on the total amount. There is also no limit on the number of different IRA accounts receiving the deposits, although there is generally no reason to use more than one. You can deposit $8,000 in February and, if you timely request a filing extension (Form 4868), you can contribute deposit $2,000 in August as long as the gross amount of your code-M distribution was at least $10,000. Any portion of the code-M distribution that remains undeposited by the deadline will remain a taxable distribution and, if box 7 of the Form 1099-R also includes code 1, subject to a 10% early-distribution penalty on the taxable amount.
