The Beach Boys had five founding members.
The original group consisted of brothers Brian Wilson, Dennis Wilson, and Carl Wilson; their cousin Mike Love; and their friend Al Jardine. This five-person lineup formed in Hawthorne, California, in 1961 and provided the core identity of the band.
Each founder initially contributed something different. Brian became the principal songwriter, arranger, and producer; Dennis played drums; Carl played guitar; Jardine played rhythm guitar; and Love became a prominent lead vocalist. All five also sang, making layered harmony central to the band's sound from the beginning.
The number five refers to the founding lineup, not every musician who later performed with the Beach Boys. David Marks joined soon after Jardine temporarily left, while Bruce Johnston became an important later member and substitute for Brian on the road. Those later additions can make discussions of the band's “original members” seem inconsistent, but the standard founding count is five.