Crop rotation with varying quantities

I understand the principle of crop rotation but I don't have an equal amount of each type of veg to rotate.  I only have one patch of onions and associated but two beds each of brassicas and root veg.  How would this work?

You can group some crops together eg. aliums with root vegetables and salad crops. Count these as one group for rotation purposes.

I think the squash family (including courgettes) aren't fussy either

Good point, sweetcorn can go on the list too!

I understand the principle of crop rotation but I don't have an equal amount of each type of veg to rotate.  I only have one patch of onions and associated but two beds each of brassicas and root veg.  How would this work?

Depends how much time, energy and space you have, but this is what we do - 1st arrange things so the biggest crops have enough room , then make every section of the rotation that size (so for example in your situation, the brassicas need 2 beds, so every bit of the rotation gets 2 beds each & for a 4 year rotation you make 8 beds). Then of course you find there are gaps in the beds for less popular things. We fill those gaps with green manure, cut flowers, herbs, flowers for pollinators, Mrs Windy's dye plants . As mentioned above there are other things that don't need to rotate, that can fill the gaps.