In addition to what DWhatley said, I would also recommend doing your best to get them individual enclosures if you intend to keep as many alive to adulthood as possible. Juvenile octopuses can and will kill each other, and sometimes they may seem fine in a tank and then all of a sudden half the population will be dead overnight. I've seen a tank of 22 individuals be reduced to 5 after one particularly bad night.
If you are determined to keep the individuals alive I would plan on setting up a seawater system for them as soon as you can, with a sump and a manifold that leads to your different individual tanks, and a drain system to bring all that water back to the sump.
I've seen two main schools of thought when it comes to tank sizing for growing octopuses. The strategy you employ will heavily depend on your individual situation.
1) You can start each animal in a small tank (something like a retrofitted bug enclosure or tupperware-esq container) and then gradually move them into progressively larger tanks as they grow. This has a few advantages because you have smaller tanks to clean, and it is easier to find and feed the octopus. Keep in mind that you will have to spend more time moving the animals to larger tanks when thy get larger, as well as purchasing/making many differently sized tanks. (I usually do three different sized tanks for the octopuses that I raise)
2) You can start each animal in a fully sized tank. This is nice because you don't have to move the animal to a larger tank when it grows out of the small tank, but requires a larger system and you don't know how many babies will actually survive to the size where they will need the larger tank, so you may waste resources buying and setting up tanks for animals that don't live longer than a few weeks. Some people also think that the larger tank the more comfortable the animal is at any size.
Some other strategy you could use to separate you octopuses initially without having to set up individual tanks for every one is to buy something like a bead organizer (heres an example)
and drill 1/2" holes in the bottom of each bead compartment, and then glue screen over the entire underside of the bead organizer. Then you can put each octo in their own compartment with a shell as a den and you will be able to separate them and keep track of them by writing numbers or identifiers on the lid of the organizer. Prop up the bead organizer in your tank near an area with good flow to make sure that they get a proper turnover rate in their enclosure.
Its important to note that some of these bead organizers don't close all the way, and animals may be able to move from one enclosure to another within the bead organizer.
This is just a cheap alternative that you should only really use for a few weeks until you have time to set up a separate system with individual tanks for each animal.
Again, this is just advice if you are really serious about having maximum longevity for each of these animals. The suggestions made by DWhatley will work much better than just leaving the babies in their original tank, and these are a few additional steps you could take in the future once they grow a bit more.