Depends how big it is - if its not large, then use John Innes No. 2, if it is already a good size, JI No. 3, or a mix of either of those with a bit of ericaceous compost. Assuming you want it in a pot, that is...

I agree with Bamboo with John Innes , most failures with acers in pots is when people use a multipirpose compost that dries out quickly over summer and hold too much water around the roots over winter.
Gg