I'm not sure, if I understood your question correctly. From your code, you're going to ask user to enter 1 sentence with length lesser then 25 characters. But you're asking for 25 sentences of arbitrarily length.
This code requests for user input, then input is limited to 25 characters. After it you can check, what types of characters is in input string.
int maxLength = 25;
int digit = 0;
var theString = Console.ReadLine();
theString = theString.Substring(0, theString.Length > maxLength ?
maxLength : theString.Length);
foreach (var ch in theString)
{
if (char.IsDigit(ch))
{
digit++;
}
}
Console.WriteLine(theString.Length);