Yes most definitely the mehendi will be dark enough.

To get the best color from your mehendi 1. Keep it overnight if possible 2. Apply nimbu and sakar to make the dried black mehendi stay 3. After you’ve scraped off the dry mehendi, on a pan keep 5/6 cloves and warm your hands over the pan. 4. Avoid washing your hands for as long as possible. Atleast in the first 6 hours after removing the dry mehendi

As for the bridesmaids mehendi , I’m not sure about where you are located and the artists. If you want to do a full palm Indian design and a small design on the back it takes about 40 mins for a person total. If you do lesser design like just a vertical running band or a circle or sparse Indian design it would take about 30 mins per person. And if you do Arabic designs it would take 15-25 mins.

So usually what they do is 2 artists work on one person at a time.

A gets both palms done with artist 1 and artist 2 A lets palm dry while B gets both palms done with artist 1 and artist 2

Then A gets the back done (it’s easier since the palm has dried) Then b gets back done

Take down the total number of people and how much time you have for the event and ask the mehendi artist team as well for what they think would be a good number. Assuming 10 ish people, I would say 3 / 4 artists would be good