There is no thumb rule that you have to reach X number of tractions before you approach investors or there is no rule of percentage you achieve month on month before you reach out to investors.
Yes metrics plays vital role in justifying your idea from concept to working model or the MVP (Minimum viable product) or POC (Proof of Concept) to acceptance state and generation of revenue from the traction.
Yes when you have good traction where in again what is good is questionable so the best deciding person is you who has got the confidence from what it was during ideation stage, you can approach the investors.
Apart from metrics investors also look for the confidence you as the founder have got on the traction that you have reached which also plays a vital role in setting up things for next level.
In early stages it use to be like get first 10 customers then 100 customers before you proving your point, but the way things in market is happening its no more the rule.
The idea behind traction should be like when you have 100 customers buying your products you should have clarity how are you going to increase and in the process of learning what things you did to get 100 customers and what efforts are required for increasing that to 1000 you should be better judge of it.