Phillippe,

NB.Si is the same as COUNTIF, and can only be used to count either single cells with the whole value
equal to the desired string, or count 1 for each string that has a letter.  It cannot count doubles,
triples, etc within strings.  If you need to count doubles, etc., then you need to use the array
formula:

=SUM(LEN($C$1:$C$100)-LEN(SUBSTITUTE($C$1:$C$100,"a","")))

Note:
LEN =  NBCAR
SUM = SOMME
SUBSTITUTE = SUBSTITUE

Will count the number of a's in C1:C100, no matter where they are, how many spaces, or other
letters:

For example, with these five entries in C1:C5:

bac def
aa  ad e f
a
b
ad      a      f     a

the above formula will return 8.

If you want to find instances of "ad", then use

=SUM(LEN($C$1:$C$100)-LEN(SUBSTITUTE($C$1:$C$100,"ad","")))/LEN("ad")

and it will return 2.

If you DON'T need to count doubles, then

=COUNTIF(C1:C100,"*a*")

will return 4.

HTH,
Bernie
MS Excel MVP