As of right now there are only two three with any package support.
.Net Framework 4.5.1 = dnx451
.Net Framework 4.5.2 = dnx452
.Net Core 5.0 = dnx50
.NET Framework 4.6.0 (dnx46) will be released in the near future.
Still it depends on what you mean by 'available'. Anyone could release a new framework is they wanted to, that cat is now out of the bag. However by default dnu uses the project.json to find dependencies by looking at the main feed on nuget.org. Right now on nuget.org those are the only dnx framework dependencies.
For example take a look at the microsoft.aspnet.mvc nuget page:
https://www.nuget.org/packages/Microsoft.AspNet.Mvc/6.0.0-beta4
Dependencies
DNX 4.5.1
Microsoft.AspNet.Authorization (≥ 1.0.0-beta4)
Microsoft.AspNet.Cors (≥ 1.0.0-beta4)
Microsoft.AspNet.Mvc.Razor (≥ 6.0.0-beta4)
Microsoft.Framework.Caching.Memory (≥ 1.0.0-beta4)
DNXCore 5.0
Microsoft.AspNet.Authorization (≥ 1.0.0-beta4)
Microsoft.AspNet.Cors (≥ 1.0.0-beta4)
Microsoft.AspNet.Mvc.Razor (≥ 6.0.0-beta4)
Microsoft.Framework.Caching.Memory (≥ 1.0.0-beta4)
Edit: there also was a aspnet50 and aspnetcore50 in earlier beta versions but they have been deprecated and should be replaced with dnx451 and dnxcore50 respectively.