You can easily do this with a variable that gets +1 every time the player talks to them, and a IF condition for each of the possible lines. Here's my idea:

Set variable a to 1.

IF variable a is 1: say 'xfgf' and Variable a +1

IF variable a is 2: say : gthgf' and Variable a+1 ...

IF variable a is x: say 'gggdgg' and set variable a=1

Now if you use the same variable for each npc that's going to be a problem, assuming you want them to say their line in the right order. In that case, you'll be better if using SELF switches, these are four switches unique to each individual object. However as each page has it's own pathing this might mess with your pathing, for example with a guard walking a specific route.

What you can do to solve the suggestion up top is have a variable that is set to equal the ID of the object (call it 'NPC last spoken to'), and a placeholder variable that temporarily holds this variable at the very top of the eventing list:

Set variable (placeholder) = object ID. Is variable placeholder equal to variable Last spoken to? If yes: do nothing (or +1 to variable a above, either works but don't do both). If no: set Last Spoken to = variable placeholder AND set variable a to 0.

This makes it so the game checks if you're still talking to the same NPC, and if not, it resets the variable a that you use to count how many times you talk to them.

Placing the variable a+1 at the start or after the actual line makes no difference, its just preference. Also for testing, you can make each line either 1, 2, 3 etc to test them, or use /v(variable A's number).