Is one allowed to own a Round 7 branch candelabra?
According to most poskim the same way that we are not allowed to make a menorah of 7 branches when the branches are in a straight row, we may also not make one even if the branches are in a circle. Although it may seem that the prohibition is only regarding actually making such a menora, the poskim say that it is also prohibited to keep such a menorah. See sources. According to numerous poskim, it would however be permitted if one of the stem would be plugged up and made unusable.
Bais Yosef Y:D 141 quoting Maharam M’rottenberg, Shulchan Aruch Y:D 141-8, PIschei Teshuva ibid 14 quoting the Bechor Shor. Chacham Tzvi 60 permits it if it round, however the poskim are machmir like the Bechor Shor. See Shevet Halevi 10-129, Derech Sicha 1-pg. 267, Ashrei Ish O:CH(2) 4-9, Darcei Teshuva Y:D 141-52, 56, Gr”a IBid Y:D 141-21, Maharsham 7-54, Be’er Moshe 4-53, L’ehoros Nossan 3-48.