How many wounds does each Ripper Swarms model have in 40K 8e?
Each Ripper Swarm model has 3 wounds, as per the 8th edition codex. 

A unit of Ripper Swarms is made up of anything from 3-9 models of Ripper Swarms. Each base is a model. Regardless of what you actually put on the model aesthetically, it will have 1 base, and that based model will have 3 wounds. So a unit composed of 3 models of Ripper Swarms would have 9 wounds total, 3 wounds each. A unit of 4 would have 12 wounds total, etc. 

You can mix and match unit sizes to accommodate the number of models you have too. It's perfectly legal to run a unit of 3 Ripper Swarms and a unit of 4 Ripper Swarms in the same army.