The inmates can send all the letters they want, provided they buy stamps in the commissary for postage. The inmates without money on their books can get stamps and letter writing material for free (as indigent inmates).
https://www.inmateaid.com/ask-the-inmate/can-an-inmate-send-letters-to-a-loved-one-outside-of-the-prison-do-the-inmate-have-to-pay-to-send-the-letters-or-do-the-loved-ones-to-whom-the-letter-was-sent-pay-for-the-letter-received#answer
Accepted Answer Date Created: May 12,2019