depending on how new you are .. maybe benefit from reading this :

http://fm.en.kupaisky.com/Community/Forum/View.aspx?num=22538&searchtype=4&pagecount=0&searchvalue=dragon_boats&sort=1&category=C01&page1=1



as for when to class what .. it depends on the sailor .. and your plans for it long-term ...
sometimes it can be complicated .. which is why you should research the data for specifics


how many ? ... a FULL CREW for ONE nation is usually :

3-7 BO's
5-6 ENGR's
3-4 REP's
(1) REST (optional)
1 SEAMAN
1 SONARMAN
1 PLANESMAN
(pr) TORPMEN
(pr) MAIN GUN
(pr) 2nd GUN
(pr) AA GUN (for nations with dedicated AA)
1-2 SCOUT
4-6 FTR's
2-4 TB's
2-4 DB's
(1) MEDIC (optional)

Nice way to make it simple. 
First thing I recommend your first crew be a bb. Therefore you need:
2 bo
2 pairs gunners probably classed acc rld rld
5 engies
3 reps
1 scout

Class all of these on time. 
Worry about the rest once you're going.

 

Class on time your Sailors , only CVs truly can benifit from Delay classing Ftrs/ not classing past 1st Fighter Class and certian Nations AA gunners such as UK and IJN .


other then that making your crew set is option on how many you want Rep,Res,Eng etc. but rule of thunb is always have 14 sailors for you BB and or CV for the end teir result and spare BOs help to lvl your crew evenly .


Standard BB setup is 2-4 Reps , 4-8 Eng Always atleast 1 Scout , Res is Optional "Helps Prevent/Slow Burning Rate after Dmg and have 2 pairs of Gnrs on hand just so you can HH or AA .


CVs can be run countless ways Whore it out all Ftr/Bombers or 4/4 6/2 even Scouts are good once lvl120

SS is easy Planesmen Sonar and Torpedoman 1 or 2 depending 1-2 Rep rest Eng

P.S. US AA gnrs and AA guns suck better to use Reload Gnrs and the Normal sets of guns that coinside with the AA gun sets