You have to divide the 500 workers according to their roles,say, machinists, welders, AC mechanic, Electricians, Turner etc and then find under each category how many employees are coming and under each how many apprentices could be engaged. If you have 30 Machinists, you can have a maximum of 4 apprentices and so on.
The period of apprentice training can be decided by the employer but there should be a curriculum for that.
Apprentices need not be paid wages as per minimum wages notification but they should be paid stipend as per Central Govt. notification. For latest notification you may approach the Training Adviser, Related Instruction Centre of your District. Normally this centre will be functioning in the Govt. ITI/Polytechnic.
Apprentices engaged through standing order should be paid wages as per unskilled workers wages in the notification.
In Campco's case the Supreme Court has ruled that apprentices/ trainees engaged through provisions of Standing Orders, the PF will not be applicable. They are students by all means. However, in Ramnarayan Textile Mills Vs RPFC which is a case very similar to you with a lot of trainees who engage night shits and whom are paid production bonus etc it was held that trainees even though engaged as per standing orders will be considered for PF coverage. The basic reason is that the trainees are big in number and they do the work which a normal regular worker do.
In short, engage only trainees following a ratio to the regular workmen and in no case I would say it should be less than 1 for every 7 workers and never engage them in shifts, pay them for that, and allow them to do overtime.Moreover, you should have a training curriculum with a schedule.
21st July 2018 From India, Kannur