Why so many Deities?
It is totally sufficient to worship Krishna alone. Lord Krishna incarnates in different forms... Chaitanya Mahaprabhu, Sita-Rama, Krishna Balaram, Jagannath, Nrsimhadeva are all Krishna Himself, they are simply different forms that He appears in for different reasons.
In the Srimad-Bhagavatam, verse 1.3.28, it's explained that all these incarnations have Their source in Krishna. He's like the original candle that is used to light all other candles. They're all candles, but still there's one original. We don't worship demigods or "others"—we simply worship Krishna in various incarnations.