Here is the long version so you can see how it works (in case you want to search for other words)...
=(LEN(" "&A1&" ")-LEN(SUBSTITUTE(" "&A1&" "," to ","")))/LEN(" to ")
and here is the short version making use of information we know...
=(2+LEN(A1)-LEN(SUBSTITUTE(" "&A1&" "," to ","")))/4
Note: SUBSTITUTE is case sensitive, so if you want this to work even when the search word begins a sentence and thus starts with a capital letter, you would need to do this...
Long version...
=(LEN(" "&A1&" ")-LEN(SUBSTITUTE(" "&UPPER(A1)&" ",UPPER(" to "),"")))/LEN(" to ")
Short version...
=(2+LEN(A1)-LEN(SUBSTITUTE(" "&UPPER(A1)&" "," TO ","")))/4