When plotting out a book you should know about how many words it needs to be. Check your local library or book store to find the books in your genre.
Open one or two and find a page that is full of text, count the words and write that down,
find how many pages and write that down.
Now multiply the words on that page by how many pages are in the book.
Now round that to the nearest 5000.
This will be the approximate word count for that book.
I would check several in this manner to get an average. This average number is the target goal for your word count.
If you plan to self publish its not as big a deal but if you wish to get a publisher contract then you will notice on many of their website where some will post what is needed in certain genres, such as page length, word count and so on. Sometimes it is a range such as 160k to 190k words but some, such as most publishers for Romance, will give a much tighter word count. So it s very important.
It helps to plot out your story first in an outline. A lot of published authors use them but few new writers ever make it without one. I personally like to write down what I want to see in my book in a notepad I keep with me at all times. Then when I have enough ideas for a full book, I will write them on index cards... not a lot on the cards just enough so I know what it is. Then I will lay out those cards and put them in order. No need to fret over if they are right or not, it is not in stone and can be changed. One I have this, I try to make each card into a chapter. This makes things much easier and faster for me and it could work for you as well.
Until next time, I wish you all great ideas and success in your chosen craft.