How common is it for somebody to get rectal prolapse?

Uncommon. Full rectal prolapse is uncommon but not rare. More common in women, especially with prgenancy or birth issues. Related to lack of pelvic support; can be seen in patients with certain parasitic infections (whipworm), patients in chronic psychiatric hospitals. May be related to pelvic organ prolapse (uterine prolapse), rectocele, cystocele, solitary rectal ulcer. Treatable, may need surgery.
More common elderly. Can be related to generalized weakness of pelvic muscles, loss of perirectal fat which provides some cushoin at lower aspect of body. The primary factor that can be controlled to decrease risk is avoid chronic constipaition. As we get older, many common medications can make us constipated. Daily fiber or laxatives is safer than straining ones rectum out.