The archelon isn’t in the 2014 SRD 5.1; this is its stat block from the SRD 5.2.
How many archelons for a fight?
An archelon is worth 1,100 XP. Using the 2024 encounter budgets (SRD 5.2), this is how many archelons fit each difficulty for a party of four characters of the same level. A dash means even one would be over the budget.
Party level
Low
Moderate
High
3rd
—
—
1
4th
—
1
1
5th
1
2
4
6th
2
3
5
7th
2
4
6
8th
3
6
7
9th
4
7
9
10th
5
8
11
11th
6
10
12+
12th
8
12+
12+
13th
9
12+
12+
14th
10
12+
12+
15th
12
12+
12+
Budgets add up every monster’s XP, so you can swap some for other creatures of the same total. Try other party sizes and mixes in the encounter calculator.
How the archelon compares at CR 4
Against the 15 other challenge 4 monsters in the SRD 5.1, the archelon has more hit points than 9 of them, which makes it tougher than most at its CR. See every CR 4 monster.