If you're talking whole beets, they'll love 'em
Ask a local farmer for guidance on whether you should chop / halve them, and whether you need to take the tops off (some of the beet family have excess oxalic acid in the tops, which is not good for some livestock.)
As a rule of thumb, we feed whole beets (fodder beet here but they're similar), chopped, at a rate of 4lbs beet in place of 1lb concentrate. Some people winter sheep on turnips alone, no other forage, but it's a hard regime for them. Best to feed hay or silage as you would normally and feed up to 4lbs beet per head per day as a winter fillip, I'd say.
Do work out how long it'll last - you won't want your in-lamb ewes' rations to drop at any point. Better to hold off starting to feed the beets, or feed at a lower rate and top up with cake as required, than to have to stop feeding them 6 weeks before lambing...