you work this out by checking the spread of the plant as listed on the label or description. If the plant's spread is, say, 40 cm, then the planting distance between it and another the same should be 40 cm, measured from central stem to central stem. This allows for close planting - where you want space between plants, you'd increase that gap by 10 or 20 cm.
It's difficult to predict what the monkshood will produce - when I grew it, it had, as I recall, 3 stems with flowerheads, but that varies. Just use the same method to work out how close to plant. Bear in mind that, when a spread measurement is given, that means from one side to the other, so 40 cm means 20 cm one side of the central stem, and 20 the other side, all round.

as the aconitum grows year on year it will give more and more flower spikes. I have a clump 5yrs old that had 12 flower spikes last year.

Give it enough space to grow into a good clump.

Tall and slim ... 1m by 50cm.