According to the SI system (guide here: http://physics.nist.gov/cuu/pdf/sp811.pdf), acceptable options are
- 200 35 mm circles,
- 200 35-millimeter circles,
- 200 circles of 35 mm diameter.
The pertinent rules from the guide above are:
7.2. Space between numerical value and unit symbol
In the expression for the value of a quantity, the unit symbol is placed after the numerical value and
a space is left between the numerical value and the unit symbol.
The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane
angle: º, ', and ", respectively (see Table 6), in which case no space is left between the numerical value and
the unit symbol.
This rule means that:
(a) The symbol ºC for the degree Celsius is preceded by a space when one expresses the values of Celsius
(b) Even when the value of a quantity is used as an adjective, a space is left between the numerical value
and the unit symbol. (This rule recognizes that unit symbols are not like ordinary words or
abbreviations but are mathematical entities, and that the value of a quantity should be expressed in a
way that is as independent of language as possible—sees Secs. 7.6 and 7.10.3.)
However, if there is any ambiguity, the words should be rearranged accordingly. For example, the
statement “the samples were placed in 22 mL vials” should be replaced with the statement “the samples
were placed in vials of volume 22 mL.”
Note: When unit names are spelled out, the normal rules of English apply. Thus, for example, “a roll of
35-millimeter film” is acceptable (see Sec. 7.6, note 3).
10.5.3. Grouping digits
Because the comma is widely used as the decimal marker outside the United States, it should not be
used to separate digits into groups of three. Instead, digits should be separated into groups of three,
counting from the decimal marker towards the left and right, by the use of a thin, fixed space. However,
this practice is not usually followed for numbers having only four digits on either side of the decimal
marker except when uniformity in a table is desired.
Here's a worse case: "20 350 mm circles". In this case, there is still no problem, but the distinction would be very subtle:
- A small space between "20" and "350" means 20350, while a large space means 20 of the 350 mm circles.
- The context may distinguish a bunch of small circles or one big circle.