Select max int from varchar column

you need a combination because of the fact that isnumeric returns 1 for the following things

select isnumeric('+'),isnumeric('5d2') 

your where clause would be like this

WHERE VALUE NOT LIKE '%[a-z]%'
        AND ISNUMERIC(VALUE) = 1

create table #bla (value varchar(50))
insert #bla values('123')
insert #bla values('a5')
insert #bla values('789')
insert #bla values('b1')

SELECT MAX(CAST(value AS Int)) FROM #bla
WHERE VALUE NOT LIKE '%[a-z]%'
    AND ISNUMERIC(VALUE) = 1

I wrote about this here ISNUMERIC Trouble

You might try

Select MAX(BoxNumber) from {table} where IsNumeric(BoxNumber) = 1