Definitely go to an event or two and talk to racers about cars and classes you are interested in, and which ones are popular in your region. You can also look up race results to get an idea of entry numbers in recent events. Also, check out the race calendar to see how many weekends they run and when they fall during the year.
Also, be aware that Improved Touring (IT) classes are regional only. Touring is the national class that is closest to IT spec, but the cars tend to be newer and more expensive - older cars (mid 90's and older?) aren't eligible for Touring. Regional only is not bad, there are championship series' and can have great competition- that's what I raced for many years, but you can't go to the Runoffs. And note that for any national class you can always race in the regional races but also have the option to race in the national race weekends (the "Majors" and the "Hoosier Super Tour).
And here's my quick and dirty run down of production based classes (if you want the real details you should download the General Competition Rules and search for the classes below- it describes the allowed mods for each class, and has a list of all cars eligible for each class).
Touring (T2,T3,T4) - National class - Newer cars, similar prep to IT, but a few differences like no adjustable shocks allowed, and no final drive ratio changes.
Improved Touring (ITB, ITA,ITS, ITR) - Regional class, one of the least permissive for modifications with the intent of being less costly. Shocks/springs/sway bars/bushings are pretty open. Very limited engine mods - but fully built motors will take advantage of all possible things like allowable overbore, deck heads to raise compression the max of 0.5 points, fancy rings packages to reduce friction,etc. Aftermarket ECU's allowed. Must use stock transmission, can change final drive ratio, add ltd slip. All bodywork and glass must be stock, no flares or wings. small front airdam allowed. Must run stock brakes other than pads and ducting.
Super Touring (STL (Lighter mods), STU (under 2L), STO (over 2L)) - National Class. All the IT mods plus more allowances inside the motor, like custom cams. Can run lightweight flywheel. Fender flares not allowed, but can add larger front splitter, and rear wing. Some body panels like trunk lids can be replaced with lighter parts, and lexan rear windows are allowed. Can upgrade brake rotors and calipers.
Production (EP,FP, HP) - National Class - Anything from ST plus can do a lot more mods to the tub - move suspension mounting points, relocate engine, driver position, etc. Can run flares and replace most body panels with lighter parts. Lots more engine mods allowed. Engine longevity generally much less than Touring or IT. Can run race gearboxes and fancy race clutches. Can run non-DOT race tires.
Spec Miata (SM)- National Class - Very locked down mods, everybody runs same suspension. Motor allowances similar to IT, but more restricted - stock ECU, and exhaust manifold. Largest car counts, but known for lots of crash damage due to very even performance between cars, so lots of close racing and need to be aggressive to make passes, etc. SM is also the most versatile if you want to double dip and race in more than one class in a weekend. You can enter in the SM race, T4, or STL, maybe STU and conceivably be mid pack.
If you are looking or a budget option - check and see if there are any sub classes for older Spec Miata's. The NA cars from 90-96, tend to not be competitive with the NB chassis cars, but there were tons of NA cars built, so some regions have created regional classes just for the older cars. The cars tend to be much cheaper to buy than a competitive NB chassis.
Probably more than you wanted to know, lol!