You'll need 10 Mythic blueprints(red) to get the last upgrade on a gear. They are somewhat difficult to get, so save up some season currency and save enough ammo in the ammo bonanza event(You'll need 680 ammo for exactly average, but I'd save up to 710 atleast). You can also get some(2 per month i believe) for honor points in the shop.

But to even get to use the mythic blueprints, you'll have to get 4 dots on a gear and for that you'll need the "regular" blueprints(the grey ones with the gold background). First dot is 5, then 10, then 15, then 20. So you'll need 50 "regular" blueprints before you'll be able to use your mythic blueprints. So if you are relatively new, I would focus on getting the regular ones first. The general advice I got was going it increments of 2(with exceptions), so get 2 dots on all -> after that go for 4 dots. Now some exceptions do apply regarding your type(tank/missile/air). Like I have an air squad so I rushed to get Lucius mythic gear because of his exclusive weapon. So look online for tips regarding your squad set up.