How many strings does a bass ukulele have?

Four. A bass ukulele has four strings, tuned E A D G.  Because bass ukulele stings derive a lot of there bassiness from their girth, adding more strings (in the style of a 5 string bass guitar) would require a very wide fretboard.  Because 5 string bass guitars have a lower pitched 5th string ( B A D G) the extra string would be impractically thick to fit or play.  It would be possible to fit a higher pitched fifth bass string (E A D G C), but this set-up is not considered useful.  Buzzards Field supplies a kit enabling any small sized classical guitar to accommodate bass uke strings and thus become a bass ukulele.