There is a trick for this; namely substituting a null string for every "s" you find.
If your initial range of 100 or so cells is named 'data', then the named formula 'remainder'
= SUBSTITUTE( data, "s", "" )
defines the array of unwanted characters. The result you require is then given by
= SUM( LEN(data) - LEN(remainder) )
At the risk of causing massive confusion, Microsoft has just released a beta test function LET, which allows this to be written
= LET(
remainder, SUBSTITUTE(data,"s",""),
instances, LEN(data) - LEN(remainder),
SUM(instances)
)
The LET function allows the user to define names for use within a single formula.