Excel search for substring and display count

To get the count in N1 put:

=COUNTIF($A:$A,"*" & M1 & "*")

then you can use this formula to find the value with the most:

=INDEX(M:M,MATCH(MAX(N:N),N:N,0))

In one formula, with your prefixes still in M1:M3, use this array formula:

=INDEX($M$1:$M$3,MATCH(MAX(COUNTIF($A$1:$A$4,"*"&$M$1:$M$3&"*")),COUNTIF($A$1:$A$4,"*"&$M$1:$M$3&"*"),0))

Being an array it needs to be confirmed with Ctrl-Shift-Enter. If done properly then Excel will put {} around the formula.

With array formulas we want to reference only the ranges with data, and not use full column references.