Was told by our Elections Office that anyone who is on the appropriate electoral roll can propose or second as many applicants as there are seats. So if your council has five seats, the same individuals can propose or second up to five applicants but not six or more. My local elections office has stated that they will withdraw the forms if they find the same proposer or seconder for more than the number of seats available but I couldn't say if that has the backing of legislation or indeed if they are actually likely to recognise that this has happened. I'm being careful to ask mine how many they've signed!
A candidate can propose or second another candidate provided they are also on the relevant electoral roll.