The MRT PTX transponder only comes with a choice of 20 numbers. This should be ok for club racing but may clash with others numbers at larger meeting, although i very much doubt it.
The AMB personal transponder, albeit dearer, is unique to you wherever in the world you may race.
I have 1 AMB transponder and 2 AMB MRT clones, all with the same number unique to me. Clones are about £40
check out
http://www.team-mrt.com/
hth
tel