How can I have a formula that counts records from a table?

Something like this

let p := 'Your String';
cnt(select TableName where 'Your String' = p)

Hi, I did not quite get it: what is p in this respect? I mean You want to test the match of a table entry and a given string. What does where 'Your String'=p mean 

Thanx in advance

p is a variable to store the data that you are looking for. In the second line it should read

cnt(select [Table Name] where [field name] = p) <-- replacing Table Name and Field Name with your specific info. 

you can skip the first line and just go with

cnt(select [Table Name] where [field name] = "search string")

if you are only searching for one thing.