Use depth-first search (DFS) to mark all individual connected components as visited:
dfs(node u)
for each node v connected to u :
if v is not visited :
visited[v] = true
dfs(v)
for each node u:
if u is not visited :
visited[u] = true
connected_component += 1
dfs(u)
The best way is to use this straightforward method which is linear time O(n).
Since you asked about the union-find algorithm:
for each node parent[node] = node
for each node u :
for each node v connected to u :
if findset(u)!=findset(v) :
union(u,v)
**I assume you know about how findset and union works **
for each node if (parent[node] == node)
connected_component += 1
