Due to generic type erasure List class has a single implementation for all its generic instantiations. You can only get a class corresponding to List<*> type, and thus create only Builder<List<*>>.
That builder instance is suitable for building a list of something. And again due to type erasure what that something is you can decide by yourself with the help of unchecked casts:
Builder(List::class.java) as Builder<List<Number>>
Builder(List::class.java as Class<List<Number>>)
Another approach is to create inline reified helper function:
inline fun <reified T : Any> Builder() = Builder(T::class.java)
and use it the following way:
Builder<List<Number>>()