Learn Finnish: Onko teillä pehmeitä lisävuoteita?

Lisävuode is a compound word ending in vuode (bed). Words ending in -e have a basic stem with two e's, so the stem is vuotee- (notice that the consonant also changes from d to t!). To make the partitive plural, one of the e's drops out before we add the -ita ending, giving us lisävuoteita.

Why is the final ä missing in pehmeitä? The basic word is pehmeä (soft).

When forming the partitive plural, we add -ita or -itä. If the basic word ends in a vowel pair like -eä or -ea, the very last vowel drops out to make room for the plural ending. So, pehmeä loses its final ä and becomes pehmeitä.

If we are asking about multiple beds, why do we use the singular verb onko instead of the plural ovatko?

In Finnish, the "to have" structure literally translates to "at someone is". In this specific structure, the verb olla (to be) is always frozen in the third-person singular form (on), no matter how many things are owned! So, even for plural beds, we use on (or onko to ask a question).