Thanks for your query.
As per your description of a big red out raised patch and 50+ spots on the back and chest itchy at times,I feel it could be probably what we call as pityriasis rosea
Its exact cause is not known but it may be triggered by a viral infection. It is self limiting and may last for 4-6 weeks.
Other less probable possibilities are ringworm, tinea versicolor
, eczema,or psoriasis
You may consult a dermatologist
for exact diagnosis and appropriate treatment.
Hope this helps you.