I planted my bare root raspberries. The instructions said 18-24 inches apart. So I did 18 inches apart in an 8 foot raised bed (5 plants). Now I’m thinking this might be too close. But I have no experience. Never grown raspberries before.
I wouldn’t worry about it, they are going to spread everywhere anyway. Eventually they will take over the 8 foot box.
The recommendation I am going by is to dormant prune the floricanes down to 3-4 per linear foot. As Robert stated, it will eventually fill the whole bed, so the spacing will have more to do with how many primocanes they will put up in the first couple seasons. Even if each of the 5 plants put out 6 primocanes, you would not have to do any thinning in an 8 foot bed.
You will need an impermeable border to contain them, unless you want them to spread, best to build your border now, once they start spreading its a chore to stop them, they are very invasive