The level cap of some weapons is +5 whereas other weapons can be upgraded to +10. This is due to the rarity of upgrade materials. The weapons that cap at +5 are upgraded using Twinkling Titanite and Titanite scales whereas the weapons that go up to +10 require Titanite shards. Titanite shards are readily available throughout the game world whereas Twinkling Titanite and Titanite Scales are relatively harder to come by.