Six danios, and why six is both floor and ceiling
Zebra danios are schooling fish: kept alone or in pairs they turn shy, nippy and stressed, and five or six is the smallest group that settles into natural behaviour. In a 10-gallon tank that minimum school is also close to the maximum the space serves, because danios are not fish that hover — they swim, fast, for most of the day, tracing the full length of the glass in laps. A school of six in a standard 10-gallon works; it works noticeably better in the longest-footprint 10-gallon you can find, since horizontal room is the resource danios actually spend.
Length over volume, lid over everything
If you are choosing the tank, a longer, shallower 10-gallon suits danios far better than a tall cube of the same volume. Whatever the shape, a tight-fitting lid is not optional: danios launch themselves at feeding time, when startled, and sometimes for no reason an owner ever learns, and a fish that jumps is usually found too late. Leave the front of the tank as an open swimming lane and push plants and decoration toward the back and ends, so the school gets a clear racetrack with cover at the edges to dart into when something spooks it.
Cooler water than the tropical default
Zebra danios come from cool, flowing streams and are happiest in water that many tropical keepers would call chilly — roughly the mid-60s to mid-70s Fahrenheit. In a normally heated home, a 10-gallon danio tank may need no heater at all outside cold snaps, which also shapes the tankmate question: most small tropical fish want water warmer than danios prefer. Danios are hardy within their range, but pairing them with warmth-loving species means one group or the other lives at the edge of its comfort for its whole life.
Keep the danios as the only school
Ten gallons supports one school and little else. Adding a second mid-water species crowds the same swimming level and splits space that neither group can spare. A few cherry shrimp or a single nerite snail can share the bottom and the glass without competing, and that is the sensible limit here. Feed a small-particle food once or twice daily — danios are enthusiastic surface feeders — and keep up weekly partial water changes, because an active school in a small volume uses up the water quickly.
Checking the plan before the fish
A six-fish school is a small commitment to make and an easy one to get right, provided the tank shape and the lid are sorted first. For a second opinion on tank dimensions, school size or a cleanup-crew idea, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.