Good morning.
Firstly I believe the flowers have been put straight on the cake using royal icing. Make sure you make them a couple of days before so that they are dry and will hold their shape and make it easier to handle them.
When it comes to using dowels wooden verse plastic is a personal choice. The only thing to watch is splinters with the wooden ones when you are cutting them down. As to how many, just make sure each area / corner of the cake is supported. 4 or 5 around the edge and then 1 or 2 in the middle. more for large tiers and less as you go up for the smaller tiers. If you google it you can often find pictures that show you where to put them and how many to use for different sizes which can be helpful with knowing where to position them.
There are a few ways you would do the top. Oasis would be one way making sure it is not touching the cake. But if you wanted to keep it edible you could use a sphere of rice krispie treats cover in fondant (same colour as flowers) and then attach the flowers with royal icing. This would save having to wire each flower which will save you a lot of time.
Good luck with the cake, hope it all goes well and they have a wonderful day. x
CakesWorth
