I've got one custom bridge (Advanced) from Tor project. Is it enough or do I need to get more?
Tor will only use one bridge at a time, but it is good to add more than one bridge so you can continue using Tor even if your first bridge becomes unavailable.
Basically it will be nice to have backup custom bridge(s) (or you can just use the other provided available bridges) in case your main custom bridge fails. I think it will be especially useful if all the other provided bridges doesn't work for some reasons too.
For exactly how many backup custom bridges you will need depends on your threat model. Everyone have pretty different threat models catered for their situations so I can't really pin point exactly how much you will need. Maybe about 3 custom bridges will do if you are in an oppressed country but you just want to use an uncensored internet more causally? But that is just my unsupported and rather random guess. I hope it helps a bit.