Select data with a join
You could do it like this:

SELECT  CONCAT(Name,' ', Surname) as FullName,
        COUNT(purchaseId) as "Quantity of purchases"
FROM Purchases as P
INNER JOIN Clients as C 
on P.ClientID = C.ClientID
GROUP BY C.ClientID,Name, Surname
HAVING COUNT(PurchaseId) &gt; 1;


DB-Fiddle

As a remark, if Fullname or Surname can be NULL then you would have to add a check for that, could be done with IFNULL()

SELECT  CONCAT(IFNULL(Name,''),' ', IFNULL(Surname,'')) as FullName,
        COUNT(purchaseId) as "Quantity of purchases"
FROM Purchases as P
INNER JOIN Clients as C 
on P.ClientID = C.ClientID
GROUP BY C.ClientID,Name, Surname
HAVING COUNT(PurchaseId) &gt; 1;