True strike spell and others like it.

Greetings all, I'm a pathfinder player, and I'm trying my hand at 5E. I'm a bit confused on how the spell true strike exactly works in 5E. In pathfinder the spell gives a number value of a +20 to your next attack, however in 5E it just says you gain an advantage. Could someone please let me know how to determind a number value for this spell and others like it? Thank you

There isn't a numerical value. When you have advantage on a d20 roll you roll 2 d20s and count the higher of the two rolls. If you're using the standard 5E by Roll20 sheet there's an option on the Options (Cog) page of the sheet that lets you set it to roll advantage automatically (in which case you take the left side roll if you don't have advantage or disadvantage), to ask you every time you roll, or to give you a toggle button at the top of the sheet so you can turn on advantage or disadvantage. EDIT: There is a numerical value to advantage in the event that you're talking about a passive ability check (passive Perception being the most common one). In that case, you add +5 to the passive score.

By the way, True Strike is widely regarded as one of the worst spells in the game. Attacking twice is generally statistically better than giving up an entire action to gain advantage on the next one (very roughly a +5 to the roll).

Not a bad spell for a rogue to have ahead of a traditional round 1 though... high initiative and true strike allows for instant sneak attack damage ;)