Do you plant the same bulb and it grows another cluster of onions?

Yes. These originals, saying 1 order is 8-12 bulbs, each bulb will produce a cluster when planted. Those 8-12 clusters will represent quite a few bulbs. About 2 of those 8-12 clusters will be replacing yur original 8-12 bulbs and 6-10 clusters will be
the amount you have multiplied yur original stock by. Now you can do what you want, keep aside the same # of bulbs you started with for replanting for the following years' crop, and eat the rest,.... or keep even more aside for reseeding and have a really huge harvest the 2nd yr.