How many times do I need to read the play?

The cheeky answer would be "as many times as it takes". But I'm not cheeky today.

1. Just read it! Read the play once all the way through from start to finish (in one sitting if possible) to get the gist of the play. Afterwards I usually write myself a quick synopsis by act or scene to make sure I retained the plot of the play. 
2. Search for line and stage cues. The second time make notes of any line that is specific to clothing, time of day, time of year, etc. Anything that you need to know when clothing the characters. Is it early morning and raining in Georgia in August? Make note of that. What year or years does the play encompass? 
3. Learn about the characters. I usually do a little of this in the beginning too (This is a guideline, sometimes I smoosh these together). Better understand each character you are dressing. Find out their age, social rank, etc. This is much easier to do in a play such as The Little Foxes as opposed to Titanic: The Musical. In larger productions try to do this at least for the main characters. 
4. After meeting with the design team read it again. They probably mentioned something you missed. 
5. Read it, again. Just do it. You probably missed something again. 

Five could be the magic number. For the purposes of this post five is, but take it as a guideline, not a rule. If it is a play I've designed before I skip straight to #2 or #3. After meeting with the design team and director reading the play with their ideas and vision in mind also allows things to come forward that you may have overlooked the first few reads. But, when it doubt read it again, with a pad of paper and pen by your side. 

Yes, I have written about reading the script before in one of my Life Lessons from the Costume Shop and the number of times and order has changed since I've written that post. These are guidelines as I stated above. Read both, see which you prefer. 

Once you've read your play, check out this article on places to do research other than the internet!