I was stuck on w5 for some time myself.
It doesn't matter how many girls you have, it will still require some time and dedication to get the 1.5M if you are relatively new.
Try to get the Dark Lord girls, those give good amounts of money compared with the girls from the other trolls.

If you do not get enough money from them, then you could try different methods of obtaining money.
1. you can try regular battles with trolls and just sell everything you get (apart from gifts, which you use to upgrade the affection of your girls).
2. you could try getting some extra money with great pachinko x10 (this could also reduce the amount of money you have, so that's up to you to decide).

Luckily for you, you will have enough time to get the money, because it takes a while to complete W3 and W4.
All you can really do is farm girls in the mean time, and try to build a money buffer (cash in the money regularly and sell every item apart from gifts).

To give you an answer on the question, how many girls do I need: that depends on the girls you do have. Event girls and the W1 girls give not that much.
You need some of the girls dropped by trolls to really start building some wealth. I would say that if you can collect more than 20k a day, you should be able to get there with the additional earnings you can make (missions, selling items, pachinko).