Can you empower renekton any number of time in risen altar

No — you can't. Risen Altar only discounts actual Empower keyword costs, and Renekton, Brute doesn't have one — and nothing on either card grants the special permission to become Empowered more than once.

• Risen Altar reads "[Empower] costs of your units here cost [1] or [A] less." This only reduces costs printed with the Empower keyword (e.g., "Empower [2]"). Renekton, Brute's activated ability is "[1]: Give me +1 Might this turn" — that's a pump ability, not an Empower ability (his Keywords line shows Empowered/Deflect/Ganking only).
• Even for units that do have Empower, Rule 827.1.c.1 makes it "Play only if not Empowered." Once a unit gains the Empowered status, the ability can't activate again — unless a card text explicitly grants permission to be Empowered multiple times (Rule 441.1.c.1). Risen Altar grants no such permission; it's purely a cost modifier.
• Renekton's "When my Might becomes 10 or more, empower me" fires once when he crosses that threshold for the first time (and only if he wasn't already Empowered — "empower me. I become Empowered if I'm not already"). If he's already at 10+, the "become" event never re-occurs (FAQ #11768).

If you disempower Renekton (e.g., with Profiteer), then yes, you could empower him again later — but that's a separate removal/restoration of the status, not "any number of times" from Risen Altar.