A temporary increase in the number of circulating reticulocytes will most likely predictably result when an individual:

Movement of an individual to a higher altitude (with decreased paial pressure of oxygen) requires greater oxygen-carrying capacity of blood and is thus a stimulus for an increased rate of erythrocyte production. This is reflected by an increased percentage of circulating erythrocytes that are newly developed (reticulocytes). Exposure to infectious agents, vaccines or allergens will elicit responses involving distinct populations of leukocytes. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 35. Gas Transpo & pH. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong's Review of Medical Physiology, 24e.