@jeevanmd Welcome to the forum! Yes, you are correct you’d need and Eb harmonica to play blues in Bb. Similarly, you’d need an Ab harmonic to play.
There are 12 keys so to have a full set you need 12.
B is a pretty rare key, so you can probably put off F#.
Traditionally your G harmonica is the lowest key, and (because F# is rarely played) F is the highest (commonly played) key. F is very often used in country and gospel stuff, as well as blues. Because it’s so high it cuts through the mix easily, but you also might find it a little shrill sometimes, in which case picking up a Low F is worth considering, which by contrast is mellower tone (but harder to cut through the mix and requires more effort to bend notes.)
So the next keys you probably wanna consider picking up are:
Bb - plays blues in F
F - plays blues in C
Ab - plays blues in Eb
Eb - plays blues in Bb
(Of those Bb and Ab are in the lower range, and Eb and F are in the higher range.)
Hope that helps!