Not everyone can be in a couple, but you can get all characters as long as nobody dies.
I have all mothers married off and I still have a few male bachelors left over. They make sure you can get each kid.
At least three male units are destined to be foreveralones once you marry all the mothers.
This is mainly because of MU. If your MU is male, he and Chrom both take wives and leave three bachelors. If your MU is female, then there's potentially two less bachelors leftover at the end leaving just one.