50 * 19 kB is less than 1 MB. 100 GB is 100,000 MB. So, after sending those mail, you'll still have 99,999 MB (99.999 GB) available for other uses. I don't expect that will be a problem for you.
I assume that 100 GB is the amount of data per month that you can use on your phone or tablet without paying extra. Since you didn't tell how much you would have to pay for an additional MB (that depends on your subscription contract), it's impossible to answer your question, if you would surpass the 100 GB.
I want to send a one page email (19 kb) to 50 recipients. I have 100 gb of data available. Will the cost be very high?