Since 5 students from each of the 6 math classes have volunteered, there are 5*6= <<5*6=30>>30 total student volunteers.
In addition to the 30 student volunteers, 13 teachers have also volunteered so there are 30+13= <<30+13=43>>43 total volunteers.
Mr. Johnson needs 50 volunteers and currently has 43 volunteers, so he needs 50-43= <<50-43=7>>7 more volunteers.
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