If they are a licensed school and inspected by the MD State Dept of Ed they go by those school regulations.
This is directly from the State Dept of Ed rules and regs for child care centers, including pre-schools.
http://www.marylandpublicschools.org/NR/rdonlyres/FF4D42D2-46A0-44E6-9CEC-546C5641F978/31469/Sub16CTRamend012312.pdf
.03 Group Size and Staffing.
A. Assignment of Staff. One or more child care teachers shall be assigned to each group of children as needed to meet the requirements for group size and staffing set forth at §§C—G of this regulation.
B. Age of a Child. For purposes of determining group size and staff required under this chapter, the age of a child is determined solely according to the date of the child's most recent birthday.
C. Same-Age Groups. In a group of children of the same age, the following staff/child ratio and maximum group size requirements apply:
Child Ages : Staff/Child Ratio : Maximum Group Size
2 years old : 1 to 6 : 12
3 or 4 years old : 1 to 10 : 20
5 years old and older : 1 to 15 : 30
D. Mixed-Age Groups.
(1) A mixed age group with preschool children may not exceed 20 children.
(2) In a mixed age group with preschool children 3 years old or older, the staff to child ratio is 1 to 10.
(3) A maximum of six 2-year-olds may be in a mixed age group of 13 to 20 children.
(4) In mixed-age groups of varying sizes that include children who are 2 years old, the following minimum staffing levels apply:
Group Composition Group Size Minimum Staffing Level
Group includes one to three 2 year old children 7 to 10 1 staff member
Group includes four or more 2 year old children 7 to 10 2 staff members
Group includes one to three 2 year old children 13 to 20 2 staff members
Group includes four to six 2 year old children 13 to 20 3 staff members
E. Infant/Toddler Groups. In a group of children where each child is an infant or a toddler, the following staff/child ratio and maximum group size requirements apply:
Child Ages : Staff/Child Ratio : Maximum Group Size
Infants (6 weeks old to 18 months old) : 1 to 3 :6
Toddlers (18 months old to 2 years old) : 1 to 3 :9
Infants and toddlers, with 1 to 2 infants in the group : 1 to 3 : 9
Infants and toddlers, with 3 or more infants in the group : 1 to 3 : 6
F. Mixed-Age Infant/Toddler Groups. In a group where infants and toddlers are mixed with preschoolers, the following minimum staffing levels and maximum group size requirements apply:
Group Composition Minimum Staffing Level Maximum Group Size
Group includes 1 or 2 infants 2 staff members 9
Group includes 3 or more infants 2 staff members 6
Group includes 1 or 2 toddlers 2 staff members 12
Group includes 3 toddlers 2 staff members 9
Group includes 4 or more toddlers 3 staff members 9
Group includes no infants, 1 or 2 toddlers, 3 staff members 12
and 6 or more 2 year olds
G. Group Size and Staffing in Approved Educational Programs.
(1) During a school day approved by the Department, a teacher shall be assigned to each class and the following requirements apply:
Child Age : Group Staff/ Child Ratio : Maximum Group Size
2 years old : 1 to 6 : 12
3 or 4 years old: 1 to 12 : 24
5 years old or older : 1 to 15 : 30
(2) A nursery school may not exceed a staff/child ratio or group size requirement set forth at §G(1) of this regulation, except that a Montessori school that has been approved by the Department may exceed a staff/child ratio or group size requirement by no more than 1/3.
(3) When children who are 2 years old, 3 years old, 4 years old, and 5 years old are grouped together, the average age of all the children in the group as of September 1 of that school year is used to determine the group's maximum size and staff/child ratio.