ask your instructor ;-)
as i have guys who have taken 4/5 interclubs to get the adrenalin dump thing out the system and feel comfortable, others are too strong or composed to be 'novices' so will go straight into C Class....
rule of thumb for mr/mrs average 2-4 interclubs to get the nerves out
then 5 c class to move up to B
however its all up to the individual..... no one person is the same as the next. As long as both you and your instructor are honest (i.e if you have had 5 pro MMA fights and 15 ABA boxing fights and 20 kickboxing fights) and you just say you are a total novice (as its your 1st thai interclub) then you/instructor are being naughty - fair matchmaking is what its about at this stage of your 'career'
after a few months training start doing interclubs as its the first intro into competition, and well run interclubs are a safe way to get a feeling of what its all about....ie 'sparring' someone from a different gym, in a new enviroment, with a cheering crowd and you having to start to go through pre fight nerves/warmups/weight management/lack of sleep the night before due to nerves/feelings of no whating to get dropped by the first punch or kick etc etc ;-)
you can then assess where you are from there....
but this is all stuff you are your instructor should discuss as he will be able to assess your level and equally you should trust in his/her opinion on how ready you are