If you don't, you will lose all of them. Remember that turnover is costly. Some companies counter the retention problems with retention contracts. They should be aware that the most highly regarded accounting and consulting firms no longer do this. Taking hostages is not the way to go .
Most people are ethical.
They know you have invested in them, and as a headhunter I can promise you, most do not relish changing companies. It takes a lot for an employee to be disgruntled enough to walk. If you treat them decently, you will come out ahead. Most people combine these factors when making career choices, but usually one is more significant than the others. Advertise a healthy training budget.