I have to enter a validation for 28 character which allows white space and if we enter any name having more then 28 char, it should rather reject that

There are some mistakes in your pattern.

The - minus sign defines a range within a bracketed character class. So it has to be escaped or the first character if you want to match the character. When using +-\/ by intention then it matches all characters from 0x2b to 0x2f (which includes the dot which can then be removed).

The s at the end is useless.
[EDIT]
To allow white spaces it must be \s.
[/EDIT]

To check for valid lengths you have to match the whole string using ^ and $.

You have not specified which language you are using. Here is a working Perl regex:

# Edit: With white spaces
# With range from + to /
#if ($test =~ /^[A-Za-z0-9_@#&+-\/\s]{0,28}$/) {
if ($test =~ /^[A-Za-z0-9_@.#&+\-\/\s]{0,28}$/) {
    print "match";
}
else {
    print "no match";
}