Based off of the original book/light novel, I don't have an answer to the first one — the only data we get is that they encounter two Alphas in the course of the history of the book — but I can answer that they did not discuss looking for more Alphas because
There can only be one person empowered to send a message back to the past to change the future. If there's more than one, then the Mimics will always be able to have future knowledge and counter the humans. That's why Keiji and Rita had to fight to the death before killing the Alpha the final time.