NACADA  Resources - 
Determining and Planning a Realistic Advisor 'Caseload', Isaiah Vance, NACADA International Conference Presentation, 2017
Advisor Load, Rich Robbins, 2013
Effective Ways to Deal with Large Advising Loads, by Debra Applegate and Gayle Hartleroad, Academic Advising Today, 2011

Other Resources - 
Advising Caseloads: Find the Right Fit, presentation by Charmagne Herlien, Tricia Schueler, Tami Renner & Amber Tetreau-Segura, NAFSA 2017

NACADA FAQ-
Does NACADA recommend a specific student-to-advisor ratio or advisor load?

The NACADA 2011 national survey of academic advising addressed, among other things, caseload issues.  Chapter 6 is comprised of data from US members in chart format that breaks professional advisor load data down by institutional type. 

NACADA does not have a suggested ratio since advising duties vary by institution type and the purpose(s) of the advising office. The CAS Standards of Academic Advising state that "Academic advising caseloads must be consistent with the time required for the effective performance of the activity" https://www.nacada.ksu.edu/Resources/Clearinghouse/View-Articles/Using-CAS-Standards-for-self-assessment.aspx  Eric White, a NACADA past-president and former NACADA representative to CAS, states that in considering caseloads "an institution must take into account

• (1) what kinds of students are to be advised and consequently what are their advising needs (e.g, athletes, honor students, undecideds, seniors in majors, underprepared students);
• (2) Who is providing the advising (professional staff with no other responsibilities, faculty with teaching and/or research responsibilities, graduate students); and
• (3) what other responsibilities would an advisor have that might determine an appropriate 'case load.'"

You can find the median number of advisees per advisor for a full-time advisor is 296 students (as listed in the NACADA 2011 Survey).  

Additionally, the implications article written by Casey Self in response to these findings http://www.nacada.ksu.edu/Resources/Clearinghouse/View-Articles/Implications-of-advising-personnel-of-undergraduates-2011-National-Survey.aspx. In the Clearinghouse section of the NACADA web you will find the Organizational Structures Index and it is here that we can provide for you articles and additional resources to best practices for Advisor Caseloads here: https://www.nacada.ksu.edu/Resources/Clearinghouse/Organizational-Structure.aspx