How many aldol condensations?

Hint: When aldol condensation takes place between two different aldehydes then one of the aldehyde functional groups changes into an alcohol group and the chain joins into one and when the hydroxyl ion attacks the other aldehyde group, it converts into an alcohol group.

Complete answer:
Aldol condensation reaction is an important reaction and property of aldehydes and ketones in which two same aldehydes or ketones and two different aldehydes or ketones having alpha-hydrogen atom condense in the presence of a dilute alkali to form a beta-hydroxyaldehydes or beta-hydroxyketones.
When aldol condensation takes place between two different aldehydes then one of the aldehyde functional groups changes into an alcohol group and the chain joins into one and when the hydroxyl ion attacks the other aldehyde group, it converts into an alcohol group.
So, in the questions, two different aldehydes are ethanal whose formula is $C{{H}_{3}}CHO$ and methanal whose formula is $HCHO$.
The given product is:

To form the above product, three times the aldol condensation reaction will occur. First, when the ethanal reacts with methanal, a compound of three carbon atoms will be formed. In the next reaction, the formed product will be reacted with methanal, then a compound formed will be of four carbon atoms. This product will again react with methanal then six carbon atom compounds will be formed in which three alcohol groups are there and one aldehyde group is present. This aldehyde group will be converted into an alcohol group by hydroxyl ion. The reactions are given below:

So, three aldol condensation reactions will be there.

Note:
Aldol condensation reactions are of two types: self-aldol condensation reaction and cross-aldol condensation reaction as two same molecules are there in self-aldol condensation and two different molecules are there in cross-aldol condensation.