I have 2 parsons terrier (Holbeck) the bitch is in its second season and is 14 month s old .
should we wait till the third season or is it ok now?

Think it would be best to wait till at least the next season, but that is not the only consideration, have you found a suitable sire for her. Or are you using your own dog in which case is he suitable both in breed lines and the way that both the bitch and the dog are constructed, you will not want to be producing less than the best you can, you should aim for the pups to be better than both the dam and the sire. You will also wish to check out the health and temperment of both dogs ancesters.