There's no hard cap on weapon critical. But it's beneficial to have at least 60% especially for group content where there are various buffs that buff your critical damage.
By default you have 10% weapon critical
Perfect Strike passive from the Apprentice CP constellation adds 12%
Major Savagery (slotting Hunter, Flames or Oblivion, casting Green Lotus or Biting Jabs or drinking a weapon power potion) gives you 2191=10% weapon critical
Minor Savagery (have a NB in that hits a critical activates Hemorrhage passive) gives you 657=3% weapon critical
Dexterity medium armor passive gives you 328=1.5% weapon critical per piece (7.5% with 5p, 10.5% with 7p)
Precise 1H and 2H weapons give 3.5% and 7% spell and weapon critical respectively
1-4p set piece Weapon Critical bonuses are by default 833=3.8%
5p Leviathan set piece bonus is 1924=8.8%
Khajiit racial passive Carnage gives another 8%
By default your critical damage is 50% skill tool tip damage, so when you score a critical hit you will do 50% more damage
Putting points in Precise Strikes gives you up to 25% to your critical damage (realistically you will put about 56 for a 20% bonus)
Minor Force gives you 10% bonus to your critical damage - gained by Having Rearming Trap active, being linked to someone by Stalwart Guard or activating a synergy from someone using Twilight Remedy Set)
Major Force gives you 30% bonus to your critical damage - only gained from Aggressive Warhorn skill for 9.5s, almost impossible to get full up time on this even in a well coordinated group with 4 warhorns.
Templar Piercing Spear passive gives you 10% critical while you have an Aedric Spear skill slotted
Shadow mundus gives you 9% (13.7% with 7p divines) to your critical hit but it's not efficient to use it following the HotR changes - it used to give 11%.
So theoretically your critical multiplier can be as high as 1+50%*(1+25%+10%+10%+15%)=1.8 for a Templar and 1.75 for other classes. Realistically that needs to be toned down a bit. A realistic value with 56 CP in Precise Strikes 80% minor force and 60% major force up times is 1+50%*(1+20%+10%+10%*80%+15%*60%) = 1.74 for Templar and 1.69 for other classes which is achievable in a group. That means you will do 1.69x tool tip damage when you score a critical strike. Even when playing solo and not using trap your critical multiplier is still 1.6 from Precise Strikes CP alone or 1.65 for Templar. So your damage will scale quite strongly with your critical.