Yes certain functions like SUM, COUNT, COUNTA etc can accept a variable number of inputs (delimited by comma) whereas others like say COUNTBLANK etc can not (single contiguous range only) - if in doubt check out the XL Help file - they're generally (though not always) pretty good.

(also as you type in the formula you should get a rough idea of what can be added by means of the "tooltip/hint" text that appears).