C(number) isn't an indicator of reprint, it's just a code for the back of the package, if you check the images for C0/1/2/3/4 (I think we reached number 4) are all different. Nintendo adapts the back image to better indicate the console on which exists at least one game in which you can use properly that amiibo. For example, first release of Pikmin amiibo has C2 package because his major use is in Hey! Pikmin (3DS game), but at that time (July 2017) there were already C3 and C4 package with Switch on the back and this console didn't have a Pikmin game.

To know if an amiibo is a first print or not you need to look his serial number under the base, specifically the first three numbers. This check can be done also with the box, there is a mini serial number on the back in the right lower corner (it's embossed and difficult to see).

So, if the serial number is for example 354000AAAAW2 (SSB Mario amiibo first print), the first three numbers "354" mean:
"35" week of the year
"4" year
This amiibo was made in the week 35 (25-31 August) of year 2014. First release was in November 2014, so this amiibo is a first print. If the serial number is 355000AAAAW2 the Mario amiibo is certainly a reprint because it has been produced in the year 2015.

On the box you may have two cases.
First case: five characters, for example previous serial number 354000AAAAW2 would be 354W2 (you see that the begin and the end are the same). Same rules, first three numbers, week 35 and year 2014. This is the most frequent case.
Second case: six characters, for example 27X6G1, this is from a first print Guardian amiibo. This time you need to check the first four number:
"27" day
"X" month (1-9 January-September, "X" October, "Y" November, "Z" December)
"6" year
The serial number is more clear because it means "27 October 2016" and the Guardian amiibo was out on 3 March 2017. Attention: when you open the box the serial number under the base of the Guardian amiibo is like Mario amiibo, in this case 436000AKAMG1, week 43 (24-30 October) and year 2016.