Tags help customers discover your products. We highly recommend to add at least 10-15 tags that best define your product. Products with 20 tags have, on average, 2 times more sales than those with only 5 tags.
A good combination of tags are: specific words that target your model, broad words which generalize it and show its qualities or characteristics (like color, material, condition, etc). However, avoid adding unrelated tags that have little to do with the product. Use tag suggestions to add more related tags.